In the spring of last year, an SUV carrying Sen. John McCain slipped across the Turkish border into war-torn Syria for a secret meeting with rebel commanders fighting the regime of Bashar Assad.

It was a risky mission — undertaken over the staunch opposition of the U.S. State Department — but one the veteran Arizona Republican believed was necessary to deliver a powerful message of solidarity with fighters who, in his view, President Barack Obama had abandoned.

“I consider you patriots,” McCain told the assembled commanders of the Free Syria Army, according to one of those present. “I will not rest until you get the support you need.”

Come January, McCain will have newfound clout to push for that support and promote his muscular foreign policy agenda when he is expected to take over as chairman of the Senate Armed Services Committee.

It is a remarkable return to semi-power for the often irascible 78-year-old former Republican presidential nominee — and one the senator is already planning to use robustly, vowing to hold hearings, conduct investigations and "demand answers" from administration officials in order to expose what he calls the "abysmal" failures of Obama's national security policies.

"Look at the world in January 2009 and look at the world today," McCain said in a Yahoo News interview, discussing his plans. "We've had a rudderless, feckless national security policy and no strategy."

McCain has an ambitious agenda, ranging from repealing mandated sequester cuts at the Pentagon ("That's our first priority," he said) to investigating the prisoner swap that freed four Guantanamo Bay detainees in exchange for Army Sgt. Bowe Bergdahl.

But more broadly, he says, "we need to hold hearings on [the White House's] strategy, to find out what the administration is doing."

“He’s got a real bully pulpit now to speak out — and the authority to do something,” said John Weaver, a former campaign adviser. “When was the last time you had a nominee who lost [a campaign for president] who was in a position like this to torment his victor?”

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Those close to McCain say there has always been a personal edge to his sharp attacks on Obama. He has, they say, never quite gotten over his devastating loss in 2008 to a foe he privately viewed as callow and inexperienced.

“I don’t think there’s a night he doesn’t go to sleep, stare at the ceiling and think, 'How did I lose to Obama?'” said one veteran GOP strategist who has been close to McCain for years.

McCain, for his part, dismisses the analysis as "an old hobby horse," noting that he was just as forceful a critic of the way President George W. Bush and Donald Rumsfeld conducted the Iraq War in the mid-2000s.

"Back then, I was the brave maverick — now I'm an angry old man," he joked. "I think my record shows that I am used to doing what is right regardless of which party is in the White House."

"I have reached out to this president," he added. "I have tried to work with him, and I have done literally everything I can." But, he said, when he sees the "black flag" of the Islamic State flying over Fallujah, Iraq, where so many Americans had died, "it makes me passionate about trying to prevent the loss of further American lives."

For all his blunt and provocative language (he recently called Adm. John Kirby, the Pentagon’s chief spokesman, an "idiot"), it is also true that McCain’s worldview — about the need for a more assertive and confrontational use of American power — is deeply held, consistent and diametrically opposed to the prevailing thinking at the other end of Pennsylvania Avenue.

President Barack Obama speaks after a meeting with a bipartisan group from Congress, including Sen. John McCain, R-Ariz., at the White House in June. (Chip Somodevilla/Getty Images)

McCain has been relentless for years about the need for the U.S. to stand up to Russian President Vladimir Putin, whom he calls an “an old KGB colonel apparatchik.” He has also warned against the president’s plans to withdraw U.S. troops from Afghanistan, and cautioned against a U.S. nuclear agreement with Iran — demanding instead, like most of his fellow Senate Republicans, tougher sanctions against the mullahs there.

But he is most passionate in his opposition to Obama’s policies on Syria and Iraq. Mouaz Moustafa, executive director of the Syrian Emergency Task Force, a U.S. nonprofit that lobbies on behalf of moderate Syrian rebels, recalls bringing the Assad defector known as Caesar, who had smuggled out gruesome photos of regime torture, to McCain’s office last July.

McCain already had a folder with some of the photos showing scorched and emaciated bodies of detainees in Assad regime prisons. He quickly indicated his disgust that the U.S. hadn’t done more to overthrow Assad. Picking up the folder, he slammed it down on the table in front of the defector, according to Moustafa. “I’m embarrassed to be an American!” McCain shouted, startling his guests.

In the interview, McCain confirmed the account. “I look at them [the Caesar photos] on my desk every day,” he said. “They are horrifying. They remind me not to give up. These are crimes against humanity. Until the last [Syrian rebel] is standing, I will do everything I can” to help them.

McCain in the interview was especially exercised about a Wall Street Journal report that Obama had written a secret letter to Iran’s Supreme Leader Ali Khamenei suggesting cooperation in the war against Islamic State terrorists in Iraq in exchange for an agreement on that country’s nuclear program.

“We are now trying to make a deal with the Iranians who are supplying [Assad] with the weapons and training that he needs?” McCain asked in disbelief. “How can we justify this? It's as amoral as anything I have ever seen.”

A White House spokeswoman declined to comment on McCain's assertions but pointed to press secretary Josh Earnest's remarks to reporters on Thursday that there was no military cooperation between the U.S. and Iran in the fight against IS terrorists. "We won't share intelligence with them," he said.

"I'm not in a position to discuss private correspondence between the president and any world leader," Earnest said. "I can tell you that the policy that the president and his administration have articulated about Iran remains unchanged."

McCain plans to vigorously push for an expanded U.S. campaign in Syria and Iraq, including more robust arms and training for the Syrian rebels and a “no-fly zone” that would prevent Assad’s air force from dropping “barrel bombs” on his own people.

But the critical question is how many of McCain’s fellow Republicans — now in the majority — are prepared to join his fight, says Dennis Ross, a former Obama foreign policy adviser and now a fellow at the Washington Institute for Near East Policy.

In their calls for greater U.S. intervention over the years, McCain and South Carolina Sen. Lindsey Graham "were out there on their own,” said Ross. “But the question is, has the climate changed enough ... and does he have others within the Republican majority who are prepared to join him? Is McCain a solo voice, or is there going to be a symphony?”

McCain says his ranks — the “McCain-Graham wing” of the GOP — just got stronger this week (and, he said, the “isolationist wing” led by Kentucky Sen. Rand Paul got weaker) with the victories of Tom Cotton in Arkansas, Jodi Ernst in Iowa and Dan Sullivan in Alaska. He has also confirmed that he is urging his wingman, Graham, to run for president to ensure that their interventionist policies will be represented in the GOP primaries. (Graham, for his part, seemed to dismiss the talk in a Fox News interview this week, saying, "I'm nowhere near there.")

Ross says it’s too early to say whether the ground in the Senate has shifted; Mitch McConnell, the incoming Senate majority leader, who has said little on these issues, is the man to watch. But for the Obama White House, with McCain wielding a gavel, “it’s a much more complicated environment, to be sure,” said Ross.