“Oh my God, it’s John McCain!”

A U.S. Marine had spotted McCain from a distance and sprinted toward him. Sen. Chris Coons (D-DE) and his close friend, the late Republican senator from Arizona, had just stopped at the NATO security mission in Kabul, Afghanistan, to get coffee in between meetings. Soon enough, 20 more Marines came running toward them.

“Here, take a picture of me with the senator!” the Marine told Coons enthusiastically, throwing his phone to Coons.

Fifteen minutes and 50 photos later, Coons was finished. It wasn’t the first time he had witnessed U.S. troops’ admiration of McCain firsthand, and he knew it wouldn’t be the last.

“It was so fun to see soldiers and airmen and Marines acting like kids at a Beatles concert, racing to take their picture with him,” Coons said in an interview on Monday evening after voting inside the Senate chamber for the first time since McCain’s death on Saturday night. That moment in Kabul, Coons added, underscored the benefits—and the necessity—of McCain’s overseas trips. Those travels were a hallmark of his Senate career.

As McCain’s colleagues spent most of the day Monday paying tributes to him and recalling their fondest moments with the “maverick,” a depressing reality set in among those who had traveled the globe with the late senator.

“John straightened a lot of people’s spines around the world on democracy and human rights. When John McCain came to town, you better have your story straight on what you were doing to push back on threats to democracy and the rule of law,” Sen. Chris Murphy (D-CT), who went on several foreign trips with McCain, said in an interview. “The combination of the Trump ascendancy and McCain’s death is a one-two punch for America’s attempts to try to stand up to human rights abusers and democracy destroyers around the world.”

Like any senator, McCain had his “pet” issues—ones he routinely highlighted that distinguished him from other lawmakers. But McCain’s policy priorities all intersected in the name of a greater goal: the promotion of Western democratic values in countries where such principles had been routinely ignored or trampled on for decades, even centuries. In the last two years alone, McCain was an authority on human-rights abuses in Iran, the persecution of the Rohingya in Burma, the rule of law in Egypt, anti-corruption efforts in Romania, democracy in Cambodia, and much more.

He not only met with world leaders on his overseas trips; he visited with some of the most vulnerable and persecuted people. Coons recalled a trip with McCain to the Zaatari camp in Jordan where Syrian refugees were living.

“He was just fiery, passionate about supporting Syrians who, at that point, were fleeing Assad’s butchery,” Coons said.

“ He was sought after by every leader—prime ministers and kings and everyone else. They wanted desperately to say hello to him, sit down and talk to him, get his advice. ” — Sen. Jack Reed

Sen. Jack Reed (D-RI), the top Democrat on the Senate Armed Services Committee, which McCain chaired, recalled the late senator’s efforts around the world that sometimes mimicked those of an American president. Reed accompanied McCain to Vietnam, where they visited the “Hanoi Hilton,” the prison where McCain was held for more than five years during the Vietnam War.

“He was sought after by every leader—prime ministers and kings and everyone else. They wanted desperately to say hello to him, sit down and talk to him, get his advice,” Reed said in an interview, crediting McCain with opening up diplomatic relations between the U.S. and Vietnam. The senators visited a university in Ho Chi Minh City where they met with 21-year-old students who had never known the warfare that preceded their births.

“They were a new generation that was committed to positive relationships with their neighbors and with the United States. And he was encouraging them,” Reed said.

“Encourage” was a word several senators on both sides of the aisle used Monday to describe McCain’s travels during the initial turbulent months of the Trump presidency. “Reassure” was another.

“A lot of his effort and time over the last year and a half was in reassuring our allies that we are still with them and that we will still make the commitments we have made,” Coons said, noting President Donald Trump’s unpredictability and his tendency to say or do things that give U.S. allies reasons to question whether the U.S. still has their back.

At a conference in Singapore last year, Coons said world leaders sought out McCain and were asking him, almost one by one, whether the U.S. still stood with its allies and was still committed to promoting human rights. It was dubbed McCain’s “reassurance tour,” and it was a tacit rebuke of the foreign-policy goals Trump had laid out and started to execute.

McCain continued his assault on Trump’s conduct on the world stage, even as he was receiving medical treatment at home in Arizona to treat the brain cancer that ultimately took his life. He criticized the president’s summit with Kim Jong Un in Singapore and later issued a blistering statement about Trump’s deference toward Russian President Vladimir Putin after their meeting in Helsinki.

“ There’s no way any of us could substitute for the fear that he put into people who weren’t doing the right thing for democracies abroad. ” — Sen. Chris Murphy

The Washington think-tank community benefited greatly from McCain’s service over the past few decades, and those groups often aided his crusades. The National Democratic Institute (NDI), which former Secretary of State Madeleine Albright chairs, and the International Republican Institute (IRI), which McCain chaired for 25 years, recently completed a joint election-monitoring mission in Zimbabwe. McCain was invaluable to those organizations as they tried to set up such missions, and McCain himself often joined them.

“Both Senator McCain and Madeleine Albright experienced, on a very personal level, the absence of freedom in their lives—he as a POW in Vietnam and she as a refugee escaping first the Nazis and then the communists in Czechoslovakia,” Kenneth Wollack, the president of NDI, told The Daily Beast. “That shared experience influenced their world view and reinforced the close cooperation between our two institutes.”

McCain’s colleagues were at a loss for words when asked who could fill his void—or how. It wasn’t a question they were ready to answer.

“Often, I felt like I was just an appendage with John when we went to these countries because they were there to make an impression on him, not on any of the rest of us,” Murphy said. “There’s no way any of us could substitute for the fear that he put into people who weren’t doing the right thing for democracies abroad.”