Washington (CNN) You can see it in everything he attempts to do around the globe, every conflict he wades into, every crisis he refuses to concede. And as John Kerry prepares to step down as secretary of state next month, he will carry it with him just as he has for 50 years: a deep-seated belief that America -- and, indeed, he -- can solve some of the world's thorniest problems with the right mix of politics, diplomacy, perseverance and personal charm.

For better and worse, Kerry never gives up, even when everyone around him thinks he should.

He is an indefatigable optimist, a warrior -- occasionally quixotic, always gung ho -- who refuses to stop until he's made every last phone call, bent every last ear, appealed to every last world leader who might help him in his quest.

Interviews with Kerry and more than two dozen US and foreign diplomats, State Department aides, current and former senators and foreign policy experts over the course of eight months paint a picture of a modern-day leader whose reliance on personal outreach and diplomacy harks back to an earlier era. At his core, Kerry is a throwback, an old-fashioned statesman who likes to get to know his adversaries, to turn them into friends.

"I find the word respect is something that comes up frequently in my conversations around the world," Kerry told CNN during an interview as the presidential campaign was unfolding. "People think sometimes that we are arrogant or we just think our way is the only way and so forth. And I believe in our way and I trust it, but I don't think you can just shove it at somebody. I think you have to have a conversation. I think you have to work through things. And I have found that respecting people, listening to people, working with people, creating a relationship is a far better way to try and get something done, than (to) just walk into the room and say this is what you have to do, now do it. That doesn't work very well in diplomacy."

Kerry talks to reporters en route to London on his first trip as secretary of state in February 2013. The trip took him through European and Middle Eastern capitals.

Kerry talks to reporters en route to London on his first trip as secretary of state in February 2013. The trip took him through European and Middle Eastern capitals.

Kerry touches down in Rome during a hectic nine-day trip in February 2013. Kerry accompanied President Barack Obama to Europe, Israel, Jordan and several Palestinian territories.

Kerry touches down in Rome during a hectic nine-day trip in February 2013. Kerry accompanied President Barack Obama to Europe, Israel, Jordan and several Palestinian territories.

Kerry's third trip as secretary of state focused on promoting peace in the Middle East. He visited Turkey, seen here, and Egypt before heading to London for a meeting of G8 foreign ministers.

Kerry's third trip as secretary of state focused on promoting peace in the Middle East. He visited Turkey, seen here, and Egypt before heading to London for a meeting of G8 foreign ministers.

In March 2013, Kerry met with leaders of the Syrian Opposition Coalition in Istanbul and pledged once again to increase U.S. assistance. Kerry then traveled to Brussels to discuss NATO's post-2014 mission in Afghanistan.

In March 2013, Kerry met with leaders of the Syrian Opposition Coalition in Istanbul and pledged once again to increase U.S. assistance. Kerry then traveled to Brussels to discuss NATO's post-2014 mission in Afghanistan.

During a flight from Kabul, Afghanistan, to Paris in March 2013, Kerry presents a birthday cake to traveling CBS correspondent Margaret Brennan.

During a flight from Kabul, Afghanistan, to Paris in March 2013, Kerry presents a birthday cake to traveling CBS correspondent Margaret Brennan.

Kerry boards a plane in Amman, Jordan, where in May 2013 he met with leaders to discuss the ongoing conflict in Syria.

Kerry boards a plane in Amman, Jordan, where in May 2013 he met with leaders to discuss the ongoing conflict in Syria.

In May 2013, Kerry met with Russian President Vladimir Putin and visited the St. Basil Cathedral in Moscow's Red Square.

In May 2013, Kerry met with Russian President Vladimir Putin and visited the St. Basil Cathedral in Moscow's Red Square.

Kerry checks his cell phone in Geneva, Switzerland, prior to a November 2103 meeting with Iran's foreign minister and the European Union's high representative for foreign affairs.

Kerry checks his cell phone in Geneva, Switzerland, prior to a November 2103 meeting with Iran's foreign minister and the European Union's high representative for foreign affairs.

Kerry talks to reporters on a boat on the Mekong River Delta in Vietnam in December. Kerry had patrolled the muddy waters 40 years earlier as a U.S. naval officer. Kerry also visited Ho Chi Minh City and Hanoi.

Kerry talks to reporters on a boat on the Mekong River Delta in Vietnam in December. Kerry had patrolled the muddy waters 40 years earlier as a U.S. naval officer. Kerry also visited Ho Chi Minh City and Hanoi.

Kerry stands beside a barricade at the Shrine of the Fallen in Kiev, Ukraine, in March 2014. It was part of his trip to Europe in search of a diplomatic solution to the crisis in Ukraine.

Kerry stands beside a barricade at the Shrine of the Fallen in Kiev, Ukraine, in March 2014. It was part of his trip to Europe in search of a diplomatic solution to the crisis in Ukraine.

Kerry looks out a window moments before meeting with Algerian President Abdelaziz Bouteflika in Zeralda, Algeria, in April 2014.

Kerry looks out a window moments before meeting with Algerian President Abdelaziz Bouteflika in Zeralda, Algeria, in April 2014.

Kerry talks with the crew of a U.S. Air Force plane prior to departure from Addis Ababa, Ethiopia, on Friday, May 2, 2014. He was en route to Juba, South Sudan, to demand a cease-fire in the brutal civil war that has sparked dire warnings of genocide and famine.

Kerry talks with the crew of a U.S. Air Force plane prior to departure from Addis Ababa, Ethiopia, on Friday, May 2, 2014. He was en route to Juba, South Sudan, to demand a cease-fire in the brutal civil war that has sparked dire warnings of genocide and famine.

Kerry visits patients at the Fistula Clinic at St. Joseph's Hospital in Kinshasa, Congo, on Sunday, May 4, 2014.

Kerry visits patients at the Fistula Clinic at St. Joseph's Hospital in Kinshasa, Congo, on Sunday, May 4, 2014.

Kerry kicks around a soccer ball during an airplane refueling stop at Sal Island, Cape Verde, on Monday, May 5, 2014. Kerry was on his first major tour of Africa, focusing on some of the continent's most brutal conflicts.

Kerry kicks around a soccer ball during an airplane refueling stop at Sal Island, Cape Verde, on Monday, May 5, 2014. Kerry was on his first major tour of Africa, focusing on some of the continent's most brutal conflicts.

Kerry visits a replica of Captain Cook's ship Endeavour at the Australian National Maritime Museum in Sydney on August 11, 2014.

Kerry visits a replica of Captain Cook's ship Endeavour at the Australian National Maritime Museum in Sydney on August 11, 2014.

Kerry looks out over Baghdad, Iraq, from a helicopter on September 10, 2014. Kerry flew into Iraq for talks with its new leaders on their role in a long-awaited new strategy against ISIS militants.

Kerry looks out over Baghdad, Iraq, from a helicopter on September 10, 2014. Kerry flew into Iraq for talks with its new leaders on their role in a long-awaited new strategy against ISIS militants.

Egyptian Foreign Minister Sameh Shoukry, from left, Kuwaiti Foreign Minister Sabah Al-Khalid al-Sabah, Saudi Foreign Minister Prince Saud al-Faisal, Kerry, Omani Foreign Minister Yussef bin Alawi bin Abdullah, Bahraini Foreign Minister Sheikh Khaled bin Ahmed al-Khalifa and Lebanese Foreign Minister Gebran Bassil stand together for a photo of the Gulf Cooperation Council and regional partners on September 11, 2014, in Jeddah, Saudi Arabia. Kerry and regional counterparts discused forming a coalition to support an American campaign against ISIS jihadists in Syria and Iraq.

Egyptian Foreign Minister Sameh Shoukry, from left, Kuwaiti Foreign Minister Sabah Al-Khalid al-Sabah, Saudi Foreign Minister Prince Saud al-Faisal, Kerry, Omani Foreign Minister Yussef bin Alawi bin Abdullah, Bahraini Foreign Minister Sheikh Khaled bin Ahmed al-Khalifa and Lebanese Foreign Minister Gebran Bassil stand together for a photo of the Gulf Cooperation Council and regional partners on September 11, 2014, in Jeddah, Saudi Arabia. Kerry and regional counterparts discused forming a coalition to support an American campaign against ISIS jihadists in Syria and Iraq.

During a tour of the Berlin Wall memorial on October 22, 2014, in Berlin, Kerry and German Foreign Minister Frank-Walter Steinmeier, left, chat with Regina Webert-Lehmann, who in 1989 fled from communist East Germany in her Trabant car (pictured) to Hungary shortly before revolutions swept Eastern Europe. Kerry and Steinmeier met with students and walked along a still-standing portion of the wall that divided East and West Berlin and whose fall 25 years ago marked the end of the Cold War.

During a tour of the Berlin Wall memorial on October 22, 2014, in Berlin, Kerry and German Foreign Minister Frank-Walter Steinmeier, left, chat with Regina Webert-Lehmann, who in 1989 fled from communist East Germany in her Trabant car (pictured) to Hungary shortly before revolutions swept Eastern Europe. Kerry and Steinmeier met with students and walked along a still-standing portion of the wall that divided East and West Berlin and whose fall 25 years ago marked the end of the Cold War.

Kerry and U.S. President Barack Obama meet with Zhang Dejiang, chairman of the Standing Committee of China's National People's Congress, on November 12, 2014, at the Great Hall of the People in Beijing.

Kerry and U.S. President Barack Obama meet with Zhang Dejiang, chairman of the Standing Committee of China's National People's Congress, on November 12, 2014, at the Great Hall of the People in Beijing.

Kerry walks with French Foreign Minister Laurent Fabius in Paris on January 16 through a memorial to the victims killed in the attack on the satirical newspaper Charlie Hebdo.

Kerry walks with French Foreign Minister Laurent Fabius in Paris on January 16 through a memorial to the victims killed in the attack on the satirical newspaper Charlie Hebdo.

Kerry walks off the plane at Kiev Boryspil International Airport in Kiev, Ukraine, on February 5. His visit came as international pressure grew for an immediate halt to surging violence.

Kerry walks off the plane at Kiev Boryspil International Airport in Kiev, Ukraine, on February 5. His visit came as international pressure grew for an immediate halt to surging violence.

Kerry listens to a talk during the 51st Munich Security Conference in Munich, Germany, on February 8. The Ukraine conflict, ISIS jihadists and the wider "collapse of the global order" occupied the world's security community at the annual meeting.

Kerry listens to a talk during the 51st Munich Security Conference in Munich, Germany, on February 8. The Ukraine conflict, ISIS jihadists and the wider "collapse of the global order" occupied the world's security community at the annual meeting.

Kerry rides his bike on March 16, during a break in talks in Lausanne, Switzerland, about Iran's nuclear program.

Kerry rides his bike on March 16, during a break in talks in Lausanne, Switzerland, about Iran's nuclear program.

Italian Foreign Minister Paolo Gentiloni, from left, Luebeck's Mayor Bernd Saxe, German Foreign Minister Frank-Walter Steinmeier and Kerry take a boat cruise during a meeting of G-7 foreign ministers in Luebeck, Germany, on April 15. The foreign ministers met to discuss global political and security issues ahead of a G-7 summit to take place in June 2015 in southern Germany.

Italian Foreign Minister Paolo Gentiloni, from left, Luebeck's Mayor Bernd Saxe, German Foreign Minister Frank-Walter Steinmeier and Kerry take a boat cruise during a meeting of G-7 foreign ministers in Luebeck, Germany, on April 15. The foreign ministers met to discuss global political and security issues ahead of a G-7 summit to take place in June 2015 in southern Germany.

Kerry speaks with China's State Councilor Yang Jiechi at the Diaoyutai State Guest House on May 16 in Beijing. Kerry is urging China to halt increasingly assertive actions it is taking in the South China Sea.

Kerry speaks with China's State Councilor Yang Jiechi at the Diaoyutai State Guest House on May 16 in Beijing. Kerry is urging China to halt increasingly assertive actions it is taking in the South China Sea.

Kerry is welcomed by Russian President Vladimir Putin at the presidential residence of Bocharov Ruchey in Sochi, Russia, on Tuesday, May 12.

Kerry is welcomed by Russian President Vladimir Putin at the presidential residence of Bocharov Ruchey in Sochi, Russia, on Tuesday, May 12.

U.S. Secretary of State John Kerry shakes hands with Nigerian President Muhammadu Buhari on Wednesday, May 29, shortly after Buhari's inauguration in Abuja, Nigeria.

U.S. Secretary of State John Kerry shakes hands with Nigerian President Muhammadu Buhari on Wednesday, May 29, shortly after Buhari's inauguration in Abuja, Nigeria.

It's an approach that's paid off -- sometimes.

Kerry's dogged persistence helped secure a landmark nuclear deal with Iran and an international agreement on climate change.

Photos: From Vietnam to Syria: John Kerry's foreign battles Photos: From Vietnam to Syria: John Kerry's foreign battles US Secretary of State John Kerry at the State Department. Here's a look back at his long career as a public servant. Hide Caption 1 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry, 27, testifies about the Vietnam War before the Senate Foreign Relations Committee in Washington, April 22, 1971. Eventually, he would chair that committee. Hide Caption 2 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry gives the peace sign during a silent demonstration of Vietnam Veterans Against the War in 1972 at Arlington National Cemetery. Hide Caption 3 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles On November 6, 1984, Kerry is elected a US Senator from Massachusetts. He would remain in the Senate until succeeding Hillary Clinton as secretary of state in 2013. Hide Caption 4 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry meets locals in a diner in Derry, New Hampshire, shortly after announcing his 2004 candidacy for president. Kerry would win the Democratic nomination, but lose the general election to President George W. Bush. Hide Caption 5 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry talks with Charles Rivkin, then US ambassador to France, and Martin Indyk, then special envoy for Israeli-Palestinian negotiations, in 2013 in Tuileries Garden in Paris. Hide Caption 6 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry, who played soccer at Yale University, kicks a ball during a refueling stop in Vienna in 2013. Hide Caption 7 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry meets with Saudi King Abdullah bin Abdulaziz al Saud at the Royal Palace in Jeddah in 2014. Hide Caption 8 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry listens as Israeli Prime Minister Benjamin Netanyahu makes a statement to the press before a meeting in 2014 in Jerusalem. Hide Caption 9 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Sheikh Abdullah bin Zayed Al Nahyan, United Arab Emirates foreign minister, points out highlights at the Sheikh Zayed Grand Mosque as he takes Kerry on a tour in Abu Dhabi in 2015. Hide Caption 10 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry speaks with Russian President Vladimir Putin, left, and Foreign Minister Sergey Lavrov at the United Nations in 2015. Hide Caption 11 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry sometimes takes his bicycle with him when he travels. In March 2015, he goes for a ride in Lausanne, Switzerland. Hide Caption 12 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry, Afghanistan President Ashraf Ghani and Afghanistan Chief Executive Abdullah Abdullah share a moment during a 2015 dinner hosted by the State Department in Washington. Hide Caption 13 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry talks with his staff in 2015 in Paris. Hide Caption 14 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry, on a park bench near the Imperial Hotel in Vienna, places a call to the president of the Dominican Republic in 2015. He needed crutches after breaking his leg in a May 2015 bike accident in France. Hide Caption 15 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry appears on "The Late Show with Stephen Colbert" in New York City in 2015. Hide Caption 16 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry shakes hands with Cuban President Raul Castro before a state dinner at the Revolution Palace in Havana in March 2016. Hide Caption 17 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry, at the Hotel Nacional in Havana in March 2016, calls Belgian Foreign Minister Didier Reynders to express his condolences following a terrorist attack in Brussels. Hide Caption 18 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry holds granddaughter Isabel Dobbs-Higginson, 2, as he signs the Paris Agreement to limit global warming in April 2016 at the United Nations. Hide Caption 19 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry talks to Japan's foreign minister, Fumio Kishida, during a ceremonial dance at the Itsukushima Shrine on April 10, 2016. They joined Britain's foreign minister, Philip Hammond, left, and Italy's foreign minister, Paolo Gentiloni, right, in a visit Japan's Miyajima Island during a break in G7 meetings in nearby Hiroshima. Hide Caption 20 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry meets with Russian Foreign Minister Sergey Lavrov on July 15, 2016, in Moscow. Hide Caption 21 of 22 Photos: From Vietnam to Syria: John Kerry's foreign battles Kerry prepares to board a US Air Force C-17 Globemaster at McMurdo Station, Antarctica, on November 12, 2016, en route to New Zealand. Hide Caption 22 of 22

But it didn't end widespread suffering in Syria or bring peace to Israelis and Palestinians. Kerry is spending his final days in office at the center of the fallout from last week's vote by the United Nations Security Council to back a resolution condemning Israeli settlement construction. Israeli officials accuse Kerry and his staff of pushing for approval of the resolution -- something the Obama administration has denied.

Yet Kerry is unrelenting in the closing chapter of his tenure. He pledges to work "until the whistle blows" to stop bloodshed in Syria, end violence in Yemen and put the Mideast peace process on a more promising trajectory. He delivered a speech Wednesday outlining the Obama administration's vision for the region, saying a two-state solution is "the only way to achieve a just and lasting peace between Israelis and Palestinians."

He also defended the administration from criticism in the wake of the UN vote and outline the Obama administration's vision for peace in the region.

"No American Administration has done more for Israel's security than Barack Obama's," Kerry said. "We have consistently supported Israel's right to defend itself, by itself, including during actions in Gaza that sparked great controversy. Time and again, we have demonstrated we have Israel's back."

He does this knowing that any progress he makes -- that much of what he has done in four years as secretary of state -- could be upended when Donald Trump is sworn in January 20 as president.

Trump's surprising win

Of course, this isn't how Kerry expected things to go. He was reasonably confident that a Hillary Clinton presidency would enable him to hand the State Department to a successor with a similar worldview, someone who would leave his biggest achievements intact.

But Trump's surprising election upset that plan. The President-elect has made no bones about his desire to abandon or renegotiate two of Kerry's signature accomplishments: the nuclear accord with Iran and an international agreement on climate change. He has signaled strategies on Syria, Russia, Israel and China that are diametrically opposed to Kerry's own.

In nominating ExxonMobil CEO Rex Tillerson to succeed Kerry, Trump has chosen a secretary of state who is vastly different in both style and substance from the person he intends to replace. The corporate czar is accustomed to making deals, not building alliances. And his close ties to Russian President Vladimir Putin raise questions about whether he will continue Kerry's efforts to bring peace to Syria or allow Moscow to dictate what happens there.

Since the election, Kerry's conversations with world leaders have been rife with anxiety-laden questions about how Trump will deal with hot-button global issues.

Last month, Kerry himself warned against boiling down complicated international issues to "little pithy tweets ... that pretend to somehow deal with the complexity of this age." He didn't call out Trump by name, but the message was unmistakable: Trump's often-flippant, 140-character communiqués are the exact opposite of the hands-on, painstaking diplomacy and careful study of the world for which Kerry is known.

Indeed, Kerry's penchant for diplomacy was shaped by what he regarded as a lack of it in his younger days.

The influence of Vietnam

Vietnam is the starting point, the place where Kerry's worldview was formed.

JUST WATCHED Kerry testifies against Vietnam War in 1971. Replay More Videos ... MUST WATCH Kerry testifies against Vietnam War in 1971. 00:47

A Navy lieutenant, he did two tours of duty there, earning a Bronze Star, a Silver Star and three Purple Hearts.

But the war didn't sit well with the former Swift Boat skipper. Repulsed both by what he saw in the Mekong Delta and what he witnessed in Washington after returning home, Kerry went on to become one of the most famous anti-war activists. The war, he said, was barbaric, and the politicians in charge of it feckless liars who went so far as to falsify body counts to make it look like the United States was winning.

In 1971, he testified before the Senate Foreign Relations Committee -- the same panel that he would chair 38 years later -- and famously said: "How do you ask a man to be the last man to die for a mistake?"

That philosophy guides Kerry to this day. In 28 years in the Senate and four as secretary of state, Kerry has worked to prevent the kind of mistakes that killed his friends, and that could have killed him, so many years ago.

"My time in Vietnam was defined in large measure by political decisions, by decisions politicians in Washington made or didn't make about how we would fight the war and why we were fighting the war," Kerry said during the interview with CNN. "And I learned that people in positions of responsibility have a huge obligation to young men and women in uniform to make sure, if they are sent into harm's way, we absolutely know what we are doing, why we are doing it, how we are going to do it and that we are going to win. And that we can't waste lives.

"So I came back with a great sense of responsibility," he said. "And that guides me. Not everything looks like Vietnam. I understand that. But it guides me in the fundamental approach to the facts you need, the facts you need to get. And it impresses on me a very deep sense of responsibility to future generations, to people today to get it right."

'It doesn't get better than this'

To Kerry, getting it right means nothing less than exhausting diplomatic options to solve the world's most intractable problems.

He didn't wait until he was America's top diplomat to get started. In his nearly three decades on the Senate Foreign Relations Committee, he traveled the world, focusing on issues such as humanitarian aid, nuclear nonproliferation, climate change, international drug trafficking and money laundering.

During President Barack Obama's first term -- when Hillary Clinton was secretary of state and Kerry was Foreign Relations Committee chairman -- he served as a diplomatic troubleshooter, traveling to Pakistan to smooth relations after the United States killed Osama bin Laden and to North Korea to try to prevent a nuclear agreement from falling apart. In 2009, he pursued an unsuccessful effort to enlist Syrian President Bashar al-Assad in a region-wide peace initiative.

Once he became secretary, he told CNN, "It doesn't get better than this."

If Clinton's utility to Obama stemmed from her global fame, Kerry's has come from pure sweat equity. His supporters and critics alike describe him as tireless, obsessive, undisciplined and -- perhaps most important -- unafraid to fail. They say he leaves no stone unturned in pursuit of American foreign policy, even if his efforts are sometimes misguided and unfocused.

That kind of talk doesn't faze Kerry.

"You have to have some kind of vision that is going to break the mold," he said in New York on the eve of his final UN General Assembly this fall. "Otherwise, you aren't going to change things."

Kerry Doctrine

The Kerry Doctrine is to boldly pursue historic, diplomatic resolutions to the world's toughest conflicts, leveraging long-term relationships with international leaders and employing an infinite amount of patience, persistence and audacity to make a lasting difference.

At 73, Kerry keeps up a schedule that wearies aides half his age. Since 2013, he has traveled close to 1.38 million miles -- more than any of his predecessors -- and visited 91 countries in 585 days. Much of that time has been spent negotiating high-stakes diplomatic deals involving Iran, Syria and the Middle East.

Kerry has had a flatter set of priorities than his predecessor. In her four years on the job, Clinton traveled to 112 countries in an effort rebuild America's image, which were damaged by war and torture under President George W. Bush. In addition to handling diplomatic efforts during the Arab Spring and negotiating a ceasefire between Israel and Hamas in Gaza, she pushed an expansive agenda on issues such as women's rights, Internet freedom and food security.

Though Kerry's focus has been narrower, his hard-charging efforts actually led to more diplomatic breakthroughs.

None was as consequential as the 2015 nuclear accord.

JUST WATCHED Kerry: Today a 'critical milestone' in Iran nuke deal Replay More Videos ... MUST WATCH Kerry: Today a 'critical milestone' in Iran nuke deal 01:25

For more than two years, Kerry immersed himself in talks with Iran and five other world powers, spending 18 straight days in Vienna in the final stretch. While the negotiations centered on limiting Iran's nuclear-power program in exchange for lifting economic sanctions, Kerry hoped -- naively to some officials, even in his own department -- that the deal could lead to cooperation on other pressing issues, such as fighting ISIS and ending the Syrian civil war.

Kerry quickly struck up a close relationship with Iranian Foreign Minister Javad Zarif. Both faced criticism at home when pictures of their strolls in Geneva were plastered across social media. Nevertheless, the two exchanged email addresses and private phone numbers to communicate one-on-one. During tough stages of the negotiations, Kerry spoke more with Zarif than with US allies.

Their relationship didn't prevent undiplomatic confrontations. Once, Kerry and Zarif could be heard shouting by those outside the negotiating room. Twice, Kerry walked away from the talks.

Privately, other ministers groused that Kerry was too involved, that he gave up leverage by immersing himself in the nitty-gritty issues commonly left to the staff. In the end, foreign ministers from all five of the US allies involved said the deal never would have come together without Kerry's energy and personal investment.

Turning the page

Like so much in Kerry's life, the Iran accord was influenced by Vietnam.

Outside his office at the State Department hangs a painting of the same kind of riverboat Kerry piloted during the war, a gift from former Defense Secretary Chuck Hagel, a fellow Vietnam veteran and former Republican senator who served with Kerry on the Foreign Relations Committee.

"I think when he sees this picture it reminds him of the awesome responsibility not to make decisions that have long-term consequences," Hagel told CNN. "There is no question if you go through a war and you see the suffering and devastation and consequences of war, you carry it with you. There wasn't a day as secretary of defense I didn't think of the dead we sent home and I know it conditioned John."

When I was 22, I went to war, and it became clear to me that I never wanted to go to war again."

It's not only the war itself that influenced Kerry, but what came later.

In the mid-1990s, he and Republican Sen. John McCain -- who spent more than five years in Vietnam as a prisoner of war -- led the successful US effort to reopen diplomatic relations with Hanoi. The experience of turning the page on a onetime enemy resonated deeply with Kerry.

"When he looks at Iran, he doesn't see a country that is an implacable foe," said Frank Jannuzi, who worked for Kerry in the Senate and now heads the Maureen and Mike Mansfield Foundation, which promotes better US-Asia relations. "He sees a country we have strategic differences with but where, through diplomacy, we can narrow these differences and someday restore a normal relationship."

So it was that in July 2015, as he and his fellow foreign ministers gathered in Vienna to sign the Iran nuclear agreement, Kerry explained his half-century journey from fighter to peacemaker.

"When I was 22, I went to war," Kerry said. He choked up. "And it became clear to me that I never wanted to go to war again. That's what this is all about. Trying to solve these matters through diplomacy and peaceful means."

When he finished, the small room erupted in applause, recalled Wendy Sherman, the lead US negotiator. Many of the diplomats -- including the Iranians -- had tears in their eyes.

A confident diplomat, unafraid to take risks

Clinton came to Foggy Bottom still harboring presidential ambitions, which many believed made her cautious.

When Kerry succeeded her, his political career ended. So he was not afraid to fail.

"I lost the presidency of the United States. So you can't lose anything much bigger than that. And what it taught me is, don't worry about it," Kerry told CNN. "If you have an opportunity to get things done, go get them done."

Photos: Photos: John Kerry through the years John Kerry through the years – Israeli Prime Minister Benjamin Netanyahu, right, speaks with U.S. Secretary of State John Kerry during the annual ceremony for the Holocaust Remembrance day at the Yad Vashem memorial on April 8, 2013, in Jerusalem. Hide Caption 1 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry speaks to New Hampshire students in 2003 early in his campaign for president. He won the nomination but lost in the general election to President George W. Bush. Hide Caption 2 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry with other military personnel in an unspecified location in the 1960s. During his naval service in the Vietnam War, Kerry was awarded the Bronze Star for saving the life of a serviceman as an officer aboard a patrol boat in the Mekong Delta. In total, Kerry received a Silver Star, a Bronze Star with Combat V and three Purple Hearts for his service in combat. After he was awarded the Purple Hearts for minor injuries, Kerry's request for reassignment Stateside six months early was granted. Hide Caption 3 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry listens to Sen. Ted Kennedy as he speaks to Vietnam veterans during a demonstration by Vietnam Veterans Against the War in Washington in May 1971. Hide Caption 4 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – John O'Neill, left, and Kerry, both decorated war veterans and lieutenants junior grade in the Navy and commanders of swift boats that policed the rivers of Vietnam, discussed their opposing views of the Vietnam War with host Dick Cavett, center, on September 24, 1971. Hide Caption 5 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Vice President George H. W. Bush swears in Kerry after he won his first Senate seat, as Kerry's daughters Alexandra, second left, and Vanessa look on in 1985. Hide Caption 6 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry and Rep. John McCain appear on the TV program "Face The Nation" on April 21, 1985. Hide Caption 7 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry addresses the Democratic National Convention in 1988. Hide Caption 8 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry and Vietnamese Deputy Foreign Minister Le Mai, second from right, talk with reporters after the arrival of Kerry, Sen. Hank Brown and Sen. Tom Daschle on November 16, 1992, for talks with the Vietnamese government on the fate of U.S. servicemen still missing after the war. Hide Caption 9 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry looks over a map with Nguen Xuan Phong of the Vietnamese Foreign Ministry while flying over the Mekong River Delta on November 20, 1992. Hide Caption 10 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry prays during an early Mass on January 11, 1994, at the Bei Hai Cathedral in Beijing. He paid his respects to Tip O'Neill, former speaker of the House of Representatives, whose funeral would be held later in the day in Washington. Hide Caption 11 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry stands with congressmen and Vice President Al Gore as President Bill Clinton announces his intent to normalize relations with Vietnam in 1995. Hide Caption 12 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Clinton speaks to Kerry at a Democratic leadership meeting in Washington on August 8, 1996. Hide Caption 13 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry walks the floor of the Democratic National Convention on August 15, 2000, at the Staples Center in Los Angeles. Hide Caption 14 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry and Sen. Joe Lieberman greet supporters on April 18, 2002, on Capitol Hill after the Senate voted to prevent further debate about initiating new oil exploration in the Arctic National Wildlife Refuge in Alaska. Hide Caption 15 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry speaks to local businesswomen on November 17, 2003, in Des Moines, Iowa. Hide Caption 16 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry answers a question from an audience member while former Vermont Gob. Howard Dean listens at a forum of AARP members on November 18, 2003, in Bedford, New Hampshire. Hide Caption 17 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry jokes with Iowa Gov. Tom Vilsack before speaking to supporters at Jerry's Restaurant on December 14, 2003, in Mt. Plesant, Iowa. Hide Caption 18 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry arrives at Waypoint, a center for women, children and families, during a campaign stop on December 22, 2003, in Cedar Rapids, Iowa. Hide Caption 19 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry and a veteran embrace at a town hall meeting at the Black Hawk County Soldiers Memorial Hall on January 13, 2004, in Waterloo, Iowa. Hide Caption 20 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry takes to the ice for a game of hockey with firefighters, the Boston Bruins, members of the U.S. Women's National Team and local high school players on January 24, 2004, in Manchester, New Hampshire. Hide Caption 21 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry travels in a helicopter to a town hall meeting in Jaffrey, New Hampshire, on January 26, 2004. Hide Caption 22 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry and running mate John Edwards wave from their bus at a rally stop on July 30, 2004, in Scranton, Pennsylvania. Hide Caption 23 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry gives a statement during a hearing of the Senate Environment and Public Works Committee on Capitol Hill on October 27, 2009. Hide Caption 24 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry talks to reporters after a closed session about the new START Treaty, a ratification of a nuclear-arms treaty with Russia, in the Old Senate Chamber at the U.S. Capitol on December 20, 2010. Hide Caption 25 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Secretary of State Hillary Clinton testifies before, from right, ranking member Sen. Richard Lugar, committee chairman Kerry and Sen. Barbara Boxer during a hearing before the Senate Foreign Relations Committee on March 2, 2011. Hide Caption 26 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – President Barack Obama shakes hands Kerry after announcing his nomination to be secretary of state on December 21 in Washington. If confirmed, Kerry will replace retiring Secretary of State Hillary Clinton early in 2013. Hide Caption 27 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Sen. John Kerry, D-Massachusetts, is sworn in as secretary of state on Friday, February 1. Hide Caption 28 of 29 Photos: Photos: John Kerry through the years John Kerry through the years – Kerry gestures to Japanese Foreign Minister Fumio Kishida (R) during their joint press conference in Tokyo on April 14, 2013. Kerry traveled to Asia to discuss nuclear tensions on the Korean peninsula and secured vital support from China to help defuse the weeks-long crisis. Hide Caption 29 of 29

When aides tell him something won't work, Kerry views it as an invitation to try. His motto could be summed up like this: We have a plane. Why not use it?

"If you tell him he is running into a brick wall, he will go running into the wall," said one senior administration official who has worked closely with Kerry.

That tolerance for risk, coupled with his unbreakable confidence, drove Kerry's bid within weeks of taking office to forge a peace deal between the Israelis and Palestinians. Few people believed the conditions were ripe for a deal, including the President. But Kerry argued it was critical to try. Given the tumult in the region, he believed Israelis and Palestinians didn't have much longer to reach agreement on a two-state solution.

Kerry came to the job with longstanding relationships with both Israeli Prime Minister Netanyahu and Palestinian Authority President Mahmoud Abbas that he planned to parlay into diplomacy.

"He had the notion it would be settled and settled quickly, with strong US leadership and involvement and his good standing with the parties," said Salam Fayyad, then the Palestinian prime minister, said in an interview.

Fayyad thought Kerry's plan to get a peace deal within a year was destined to fail, given the mistrust between the parties.

And fail it did. But Kerry was able to coax Israeli and Palestinian negotiators to the first direct peace talks in almost three years.

"He was literally defying gravity," says Martin Indyk, Kerry's former Mideast envoy. "Nobody expected he would even be able to get the parties back to the table. He did it through sheer energy and refusing to leave Netanyahu and Abbas alone."

For nine months, Kerry and his team shuttled between the Israelis and Palestinians, trying to create a framework agreement that would outline, in general terms, solutions to the core issues. The contentious negotiations were largely kept secret -- even from most people in the State Department.

Kerry's aides say they made progress on some key issues, like the borders of a Palestinian state and the fate of Palestinian refugees. But security, the status of Jerusalem and mutual recognition remained elusive.

Critics labeled the effort a vanity project, tantamount to a grail quest. Israeli Defense Minister Moshe Yaalon went so far as to accuse Kerry of "acting out of misplaced obsession and messianic fervor."

"The only thing that can 'save us' is for John Kerry to win a Nobel Prize and leave us in peace," he was quoted as saying in Yedioth Ahronoth, Israel's largest newspaper. He later publicly apologized for the comments.

After 100 closed-door meetings in half a dozen world capitals over nine months, the talks ended in April 2014.

Secretary of State John Kerry waves to officials and guests before departing on August 13, 2014 in Sydney, Australia.

"If this were a matter of optimism or drive or relationships, he would have succeeded," said Ron Dermer, Israel's ambassador to the United States. "But that was not the decisive factor."

The breakdown of the talks left Kerry disillusioned, but he takes umbrage at the notion that he failed.

"I didn't fail. We didn't fail. The United States didn't fail," a defensive Kerry told CNN. "We put what I think is still the solution on the table. But the parties failed. ... Unless they are willing to try and grab onto it and make it work, there is nothing the United States of America can do."

Even with the breakdown of the talks, Ambassador Maen Arikat, the Palestinian representative to the US, said Kerry's "tireless efforts were unmatched and his commitment to making progress was highly admired."

"Secretary Kerry has shown a genuine and sincere desire to break the deadlock between Palestinians and Israelis," Arikat told CNN. "His impact on peace efforts will be visible for a long time."

Throughout his term, Kerry met quietly with Netanyahu and Abbas in the hopes of possibly restarting discussions. But as the clock wound down, it was clear there was little appetite from either the parties or the White House.

Last-ditch effort for Mideast peace

Kerry still left open the possibility of a last-ditch effort to lay out his vision for Mideast peace and memorialize the progress that had been made during the negotiations. Doing so before the US election seemed too risky, as it might undermine Clinton, who was viewed as strong on Israel.

But after Trump's election, the administration worried a dramatic move -- such as supporting a UN Security Council resolution laying out the "parameters" of a final peace deal -- could push Trump to take a hardline stance. That, administration officials feared, could ironically impede his own stated goal of finally bringing peace to the Middle East.

Another UN resolution, this one condemning Israeli settlement activity, seemed a workable compromise. Kerry and his team had long considered Israel's continued settlement expansion one of the main reasons for the failure of Kerry's mission and the primary obstacle to peace. The White House felt strongly that, if allowed to continue unabated under a new administration, the settlements could create a reality on the ground that made a viable Palestinian state virtually impossible.

Although various resolutions circulated for more than a year, diplomats said the effort gained steam after the US election because of the consensus the incoming Trump administration would veto any action against Israel at the UN.

Last week, the White House decided it would abstain from the Security Council vote on the settlement resolution, allowing it to pass by not exercising the US veto. As Kerry prepared to deliver a speech explaining the vote and putting it into context of his overall Mideast vision, word about the US decision reached Israel. Netanyahu called Egyptian President Sisi, while Israeli officials reached out to Trump, who took to Twitter urging the US to veto. Hours later, when Sisi took the resolution off the table, it wasn't Obama he called -- but Trump.

By Friday morning, other countries reintroduced the resolution and, with the US abstention, the measure passed. Israel has since accused Kerry of being a "covert actor" in orchestrating the vote in what Netanyahu called a "shameful ambush" of Israel by its closest ally. In a phone call with Kerry, Netanyahu said "friends don't take friends to the Security Council."

Dermer, Israel's ambassador to the US who once credited Kerry for simply getting the Israelis and Palestinians to talk, said the secretary of state will now be remembered for the UN vote.

"Regardless of John Kerry's well-intentioned efforts to advance peace, the anti-Israel resolution passed last week at the UN will become his legacy," Israel's ambassador to the US Ron Dermer told CNN. "In addition to delegitimizing Israel, fueling the diplomatic and legal war against the Jewish state, it also undermines the chances for peace."

Riding the Kerry-go-round

His very name -- John Forbes Kerry -- says something about the secretary of state. His father was a foreign service officer, his mother a member of the wealthy Forbes family and a descendant of John Winthrop, the first governor of Massachusetts. Kerry attended a Swiss boarding school and a New Hampshire prep school before enrolling in Yale University.

As a child, he spent summers in Saint-Briac-sur-Mer, France, where his grandparents -- James Forbes and Margaret Winthrop -- kept an estate. Nazi occupiers had turned the compound into a military headquarters and burned it to the ground near the end of the war. The townspeople helped rebuild it. On the 70th anniversary of D-Day, in 2014, Kerry returned to pay tribute to the town. He was welcomed as a son of France.

In his life as a public servant, Kerry's Boston Brahmin upbringing -- and his second marriage, to ketchup heiress Teresa Heinz -- cut both ways. During his 2004 presidential campaign, Republicans used photos of Kerry windsurfing, sailing and flying between vacation homes to portray him as part of the East Coast elite. The patrician Kerry had trouble connecting with voters, who opted to re-elect the more gregarious George W. Bush.

Democratic presidential candidate John Kerry (L) makes a point as US President George W. Bush listens during their third televised debate 13 October, 2004 at Gammage Auditorium, Arizona State University, in Tempe, Arizona.

In the world of diplomacy, though, Kerry's pedigree and Ivy League education seem to give him an edge. Diplomats say he has a sense of "noblesse oblige" that puts him at ease in foreign capitals (though diplomats at the US embassy in Belgium groaned when he once spoke about bicycling along the "future foie gras").

On the rare occasion he is at the State Department, Kerry is often accompanied by his dog Ben, a yellow Labrador retriever who has been in more diplomatic meetings than many of the State Department's top officials.

But more often than not, Kerry is traveling on a Boeing 757, an Air Force jet that has broken down on more than one occasion. His small, private cabin is outfitted with a secure phone to call the White House or foreign leaders and a couch that folds out to a bed. To relax, he strums his guitar. He often can be seen lurching from his cabin in jeans and an orange Yale hoodie to take pictures of a cloud or something on the ground far below.

An avid biker, he sometimes brings his own racing bike on his travels. Last year, he decided to cycle a route in the Alps that had been part of the Tour de France, but crashed on a curb and broke his leg. This summer he returned to complete the ride.

Kerry is also known for adding impromptu meetings to his trips -- detouring, for example, from Rome to Brussels and London the day after the Brexit vote -- so that a five-day trip might end up lasting two weeks and logging thousands of additional miles. His traveling press corps (sometimes affectionately) refers to the phenomenon as "Kerry-go-round."

For Kerry, personal relationships matter, and showing up is more than half the battle.

Foreign leaders aren't always excited to see him. There is a bit of, "Oh God, here comes Kerry; I'm going to have to change my weekend plans," said one European ambassador. "They don't always feel the meeting was useful, but I assure you they would be complaining if he didn't consult."

Kerry enjoys conducting business over a good meal, particularly in Paris or Rome, where he will dine with foreign ministers late into the evening.

"He is like a bon vivant," said one senior State Department official who has traveled with Kerry. "He likes life. He likes good conversation. He likes good food. He likes to see interesting things. He sort of soaks it all up."

Abdullah bin Zayed Al Nahyan, the foreign minister of the United Arab Emirates who has had his share of late-night dinners with Kerry, said the secretary's premium on personal relationships and face-to-face discussions is an old-school form of diplomacy seen infrequently in the age of email and video chats.

"He is probably one of the last of his kind," he said in an interview. "When you are a foreign minister you meet a lot of people, but you don't get to make friends with a lot of them. With John, I can honestly say I have a friend."

His years as a senator gave Kerry an amazing confidence in his ability to win people over. He often says something akin to: "Get me in the room with (insert world leader or foreign minister here) for 60 minutes and I will convince him."

"I have never met anyone who had more self-confidence in his own capacity and belief that, by force of personality and sheer will, he could create the kind of environment for successful negotiations," said Aaron Miller, a former Mideast negotiator for six secretaries of state. "But it has helped him get a lot done."

Kerry shrugs off such descriptions of himself, offering a less muscular explanation for his diplomatic successes.

"I read that John Kerry thinks if he can talk to someone long enough he can convince them. I don't believe that," he said. "There is a ripeness to diplomacy. ...And if you can push at it a bit, you sometimes can get people to see some benefit that they haven't necessarily seen. Or be able to understand why this, in fact, in the long term is actually going to work out in a positive way."

But Kerry-the-bon-vivant sometimes is faulted -- by foreign policy experts, Republican members of Congress and even some State Department officials -- for putting too much stock in personal relationships, even confusing them for positive outcomes. Exhibit A is his special bond with Russian Foreign Minister Sergey Lavrov.

'Odd couple'

They are called the diplomatic "odd couple." Kerry and Lavrov have remained friendly even as the US-Russia relationship has deteriorated to near-Cold War levels over conflicts in Syria and Ukraine and amid evidence that Russia used cyberattacks to try to influence the 2016 presidential race.

Lavrov once joked to a roomful of ministers that "only me and John are friends, not the United States and Russia."

Their relationship helped produce the Iran nuclear deal and an agreement for Syrian President Assad to give up his chemical weapons.

A suave 65-year-old, Lavrov is known as a shrewd negotiator. While he credits his rapport with Kerry for making negotiations easier, he emphasizes that there are limits to what they can do.

"Personal relationships help overcome difficult issues, provided the basics are acceptable to the governments in general. But you can't do it alone," he said in in September, speaking to CNN in Geneva as his talks with Kerry over a ceasefire in Syria dragged on toward midnight.

"You have to promote consensus in your own country. You cannot say, 'The government is not happy but we will meet and because we are friends we will have a good result.' It doesn't work that way."

Lavrov isn't loath to take a dig at his "friend." In Geneva, as the clock ticked into the early morning and Kerry disappeared for hours to consult with Washington over the agreement, Lavrov served journalists pizza and vodka -- making clear the United States, not Russia, was responsible for the delay.

Republican lawmakers and other critics say Kerry has been "played" by Lavrov, particularly when it comes to Syria. They say his refusal to concede that the United States and Russia do not share common interests there has undermined his ability to revive a ceasefire. Kerry himself has acknowledged that Russia has dragged out negotiations to help Assad gain ground on the battlefield.

"There is a cost to knocking repeatedly on the Russians' door and regularly being rebuffed," said Robert Ford, a former US ambassador to Syria.

Asked whether Kerry was naive to Russia's intentions, Lavrov reduced the criticism to the ramblings of "people in Washington who want to ruin Russian-American relations."

"It's not just John Kerry doing this against everyone else's wishes," he said. "Whatever we managed to wrap up with John, whether it's the chemical deal in Syria or the Iranian nuclear deal, it's something the US wanted. ... So I believe he is doing a great job."

Failing in Syria -- and trying again

Nothing has tested Kerry quite like Syria. For more than a year, he has flown among world capitals to negotiate with Lavrov. He amassed a Syria Support Group of more than two dozen countries, including Iran and Saudi Arabia, whose own feud is viewed as a proxy war.

Nobody is more frustrated than me."

His efforts all but collapsed in September at the United Nations General Assembly. Kerry and Lavrov had hammered out a truce days earlier in Geneva, but then Russia seized on an apparent errant US airstrike that killed more than 60 Syrian soldiers to help Assad renew an assault on the besieged rebel-held city of Aleppo.

That week, at a meeting with Syrian activists, Kerry finally ran out of optimism. Facing a barrage of pleas to use force to stop Assad, Kerry glanced at his chief of staff and the US special envoy for Syria and candidly told the group: "I think you are looking at three, four people in the administration who have all argued for use of force, and I lost the argument. Nobody is more frustrated than me."

A week later, the United States broke off talks with Russia over a ceasefire, leaving hundreds of thousands of Syrian civilians trapped in Aleppo. Only last week were they permitted to begin evacuating.

Still, Kerry has not thrown in the towel. He continues to meet with Lavrov to try to solve what he considers the worst humanitarian crisis since World War II. He does it knowing that Trump has suggested he would strike a deal with Moscow that abandons the Syrian opposition -- and likely leaves Assad in power.

Kerry is pushing ahead even though, just last week, the foreign ministers of Turkey and Iran met with Lavrov to talk about the road ahead on Syria. Kerry, unable to make promises on behalf of the new administration, was not invited.

Where is the President?

With his late-night dinners and friendships around the globe, Kerry offers a useful counterbalance to a President who is short on personal relationships with world leaders and often perceived as detached. But his can-do attitude (and, some would say, bullheadedness) also can annoy a buttoned-down White House.

"Kerry is like a missile you can fire but you can't control," said one senior administration official familiar with National Security Council discussions. "He is like a dog with a bone when he wants something. That is exasperating to some people when we have made a decision."

They might roll their eyes over Kerry's exuberance, but White House officials concede he has surpassed expectations. If there is anyone the President wants at the negotiating table, it is Kerry.

"Oftentimes, nobody takes up the hot potato. He is willing to pick it up and run with it, and it's served us well," said Deputy National Security Adviser Ben Rhodes. "There is no way you get an Iran deal without a secretary of state who is ready to sit there day after day on a hot issue without knowing he will succeed."

Yet both Kerry's supporters and critics question how much the White House supports him. Several European and Arab ambassadors recalled asking White House officials about the status of Mideast peace talks and hearing, dismissively, "That's a Kerry thing." Both Israeli and Palestinian officials said they were never certain the President was invested.

The Syria crisis laid bare the disconnect between Kerry's optimistic worldview and tolerance for risk and Obama's risk-averse approach to a region in turmoil.

Kerry "has a much more audacious and ambitious vision when it comes to using US power to solve conflict than the president he is working for," said Vali Nasr, dean of the Johns Hopkins School of Advanced International Studies. "The White House wants to do the least required, and Kerry has a broader, more maximalist view of the US role."

On several occasions, Kerry urged Obama to impose a no-fly zone in Syria, arguing that shooting down a few Syrian planes "could change Assad's calculus" and get the Syrian President and Russia to the table. But Obama rejected the idea, at one point forbidding anyone but his secretary of defense from sending him proposals for military action.

Obama's decision in the fall of 2013 to ignore his own "red line" and refuse to launch airstrikes against Assad for using chemical weapons dealt a severe blow to US credibility. It was a personal humiliation for Kerry, who had given an impassioned speech evoking the lessons of Rwanda and the Holocaust the day before the president stood down.

Even Republican Sen. Bob Corker, the current chairman of the Foreign Relations Committee who recently accused Kerry of "lying prostrate" in front of his Russian counterpart Lavrov, said the President has hampered Kerry's efforts.

"How can a secretary of state have any chance of success when the White House is unwilling, at any level, to have a backup to what he is doing if diplomacy fails?" Corker asked.

But as long as he has enough of a yellow light to keep going, Kerry doesn't seem to mind. He said Obama has given him a "long leash" and has been "quite extraordinary in having my back."

Preparing for the "roller coaster"

Kerry might be forgiven for just packing up and planning his exit. After all, anything he does now, in his final few weeks in office, can be undone by Trump and his team.

But Kerry shows no sign of slowing down. Last week, he traveled to Riyadh, Saudi Arabia, to advance negotiations on a Yemen ceasefire. And he is not ruling out more travel before his term ends.

"It will be full-f***ing throttle" until January 20, one aide said.

Another senior State Department official joked, "they will either have to pry him from the building or bring the plane back mid air."

At that point, Kerry said, he will be eager to experience life back in Boston as a "private citizen" for the first time since he launched a career in public service as a Massachusetts prosecutor 40 years ago. But it's clear that politics remains in his DNA.

Kerry told CNN he plans to work in the private sector but continue to fight for environmental and other causes, perhaps with "one foot" in the public sector. From his work to create the first Earth Day in Massachusetts in 1970, which drew 20 million people, to a failed Senate effort to limit US greenhouse gas emissions, Kerry has long been devoted to the environment. His first substantive cable to every US diplomatic post made climate change a central focus of American diplomacy and since then he has broached the issue regularly with world leaders.

Still, Kerry must face a sobering reality: When it comes to foreign relations, the new administration is likely to change course, possibly in a very big way. The international climate change agreement took effect just four days before Americans elected Trump, a candidate who called global warming a "hoax" and threatened to tear up the accord. (More recently, Trump told Fox News that "nobody really knows" if climate change is real.)

Though he criticized Trump's rhetoric during the campaign -- particularly his suggestions that the United States abandon NATO and allow Japan and South Korea to have nuclear arms -- Kerry has since adopted a wait-and-see approach to the incoming administration.

Even now, with his achievements on the line, he remains that indefatigable optimist -- expressing hope that some of Trump's nominees, including retired Marine Gen. James Mattis at the Pentagon and Tillerson at the State Department, will moderate some of the new president's stated intentions to abandon decades of US policy.

Which doesn't mean there won't be a little upheaval or, as Kerry put it, a "roller-coaster ride."

"We're going to have one hell of a debate over the course of the next few years, I assure you," Kerry told the Women's Foreign Policy Group following Trump's election. "And I can promise you this: After coming back in the 1960s and being involved in that period of time, I am not going to go quietly into the night."