Inside John McCain's 'Three Amigos' friendship
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WATCH: While on the campaign trail, Lindsey Graham and John McCain appeared together in New Hampshire trading insults and jokes.

Shortly after his brain cancer diagnosis, Senator John McCain hiked through Arizona’s Oak Creek Canyon with his daughter Meghan and two of his closest friends -– Senator Lindsey Graham and former Senator Joe Lieberman.

“The three amigos together again!” McCain tweeted at the time.

One year later, the bipartisan trio known as the “Three Amigos” is now missing its driving force.

McCain, 81, died Saturday evening.

“McCain was the central figure, really, of the three of us,” Lieberman told ABC News in a recent interview.

John McCain checks out the podium angles, along with Sen. Joseph Lieberman (center) and Sen. Lindsey Graham (R-SC), during a walkthrough before the start of the Republican National Convention at the Xcel Energy Center in St. Paul, Minn., Sept. 4, 2008.

Over his time in the Senate, McCain formed a close working relationship with Lieberman, an independent from Connecticut, and Graham, the longtime Republican from South Carolina.

They staked their claim as prominent hawkish voices on foreign policy, an issue that took them on travels around the globe and led to the cultivation of a long-lasting friendship that lasted beyond the halls of the Senate.

“We’ve traveled the world together. I’ve seen these guys in action. I’ve learned a lot from both of them and we had so much fun,” Graham told ABC News. “It was so classically American,” Lieberman, who retired from the Senate in 2013, said. “If you looked at our biographies, the three of us couldn’t be more different. Really I mean, religiously, geographically, our own experience. Yet there we were in the U.S. Senate together, and we became just the fastest of friends.”

McCain and Lieberman, then a Democrat, met in the Senate in the late 1980’s, telling each other during an encounter on the Senate floor that they hoped to work together in the future.

In the 1990’s, the two senators found themselves sharing the same views on the Bosnian war and traveling to the annual Munich Security Conference.

Senator John McCain (C) arrives with US Sen. Joe Lieberman (I-CT) and Sen. Lindsey Graham(R) R-SC at a Vets for Freedom rally on Capitol Hill, April 8, 2008.

When Graham became a senator in 2003, he joined the duo.

“The time really to get to know your colleagues is when you were fortunate enough to travel with them somewhere because you were on the plane together, you were talking, you were reading,” Lieberman, whom McCain even considered naming as his running mate during his 2008 presidential campaign, said. “That’s when I first found out of McCain’s love of Hemingway.”

After the 9/11 attacks, the trio made routine visits to Afghanistan and Iraq. It was on one of those trips when General David H. Petraeus gave the nickname to the triumvirate, which became vocal proponents of President George W. Bush’s “surge” strategy in Iraq.

“They were the three amigos. They were three inseparable friends,” Petraeus told ABC News. “At some point, I just started saying we had the three amigos coming in again.”

Serving as a Navy pilot in the Vietnam War, John McCain III, was held as a prisoner of war for years after his warplane was shot down. Upon being released, he returned home and continued to serve his country th... Photo Credit: Terry Ashe/The LIFE Images Collection/Getty Images
After the Vietnam War broke out, McCain volunteered for combat duty and flew bombing runs from an aircraft carrier against the North Vietnamese.

John McCain in military uniform at his sist... Photo Credit: Terry Ashe/The LIFE Images Collection/Getty Images
McCain narrowly escaped serious injury during an accident which killed 134 sailors and injured 161 on board the aircraft carrier USS Forrestal in July 1967.

From left, Lieutenant Commander... Photo Credit: Bettmann/Getty Images
On Oct. 26, 1967, McCain's plane was shot down by North Vietnamese over Hanoi during a bombing run. He ejected from his plane and pulled ashore by villagers after landing in lake.

Navy Air... Photo Credit: AFP/Getty Images
McCain sustained serious injuries in the crash and after being pulled from the lake was taken as a prisoner. When his captors learned he was the son of a high-ranking officer, they offered him early release, bu... Photo Credit: AFP/Getty Images
On March 14, 1973, he was released along with several other American POWs and was welcomed home by President Richard Nixon in Washington, D.C. McCain earned the Silver Star, Bronze Star, Purple Heart and Distin... Photo Credit: Getty Images
McCain returned home to his first wife Carol Shepp, whom he had married in 1965. He had adopted her two children and they had a daughter, Sidney together, But the marriage didn't survive and in 1979, he met Cin... Photo Credit: Thomas O'Halloran/PhotoQuest/Getty Images
John McCain launched his political career in 1982 when he was elected to the U.S. House of Representatives.

McCain and his wife, Cindy, address members of the press at McCain's campaign he... Photo Credit: Tom Tingle/Phoenix Gazette via AP
McCain served two terms in the U.S. House of Representatives before he decided to run for a seat in the U.S. Senate in 1986, replacing longtime Arizona senator Barry Goldwater. McCain became known as a “m... Photo Credit: Susan Biddle/The Denver Post/Getty Images
Vice President George H.W. Bush re-enacts the swearing in of Sen. John McCain with his wife Cindy and children Jack and Meghan in January 1987. Photo Credit: Cynthia Johnson/The LIFE Images Collection/Getty Images
Under federal investigation as a member of the "Keating Five," McCain and his colleagues were eventually cleared after being accused of improperly intervening with federal regulators on behalf of Char... Photo Credit: John Duricka/AP
McCain took on the tobacco industry and made campaign finance reform his signature issue, partnering with Democratic Sen. Russ Feingold to combat what he saw as the corrupting influence of large political contr... Photo Credit: CQ-Roll Call,Inc./Getty Images
John McCain and his wife, Cindy, at home in Phoenix, Ariz., with their children (clockwise from left) Jack, Bridget, Meghan, and Jimmy on Oct. 14, 1999. Photo Credit: Karin Cooper/Hulton Archive/Getty Images
In 1999, McCain published "Faith of My Fathers" with Mark Salter, which recounted his family's military history and his own time as a POW and became a bestseller.

Senator John McCa... Photo Credit: Spencer Platt/Newsmakers/Getty Images
Republican Sen. John McCain announces his candidacy for the 2000 presidential race, Sept. 27, 1999, in Nashua, New Hampshire. Photo Credit: John Mottern/AFP/Getty Images
McCain addresses a town hall meeting at a National Guard armory, on Jan. 18, 2000 in East Greenwich, R.I. Photo Credit: David Hume Kennerly/Getty Images
Republican presidential candidate Sen. John McCain, and wife Cindy, acknowledge the crowd after the Arizona senator's primary victory over front-runner George W. Bush, Feb. 1, 2000, in Nashua, N.H.
&... Photo Credit: Porter Gifford/Getty Images
Presidential candidate John McCain, his wife, Cindy, and children (from left) Andy, Jimmy, Jack, Bridget, Meghan, Doug, and Sidney, pose for a photo on Feb. 1, 2000 in New Hampshire. Andy, Doug and Sidney are M... Photo Credit: David Hume Kennerly/Getty Images
Following a devastating loss on Super Tuesday, McCain suspended his campaign and endorsed George W. Bush for the Republican nomination two months later.

Governor Bush and Senator McCain me... Photo Credit: David Hume Kennerly/Getty Images
McCain was diagnosed with skin cancer and had surgery in 2000 to remove cells on his head and arm.

Senator and Democratic vice presidential nominee Joseph Lieberman, left, and Senate Commi... Photo Credit: Paul J. Richards/AFP/Getty Images
On April 25, 2007, McCain returned to New Hampshire to announce his second attempt to win the White House. In Portsmouth, NH, he kicked off his campaign with a four-day bus tour which included stops in South Ca... Photo Credit: Justin Sullivan/Getty Images
As the presumptive GOP nominee, McCain announced his running mate, Alaska Gov. Sarah Palin at a rally in Dayton, Ohio, Aug. 29, 2008.

The presumptive Republican presidential nominee John M... Photo Credit: J.D. Pooley/Getty Images
On his second attempt, McCain accepted his party’s nomination for president at the Republican National Convention in St. Paul, Minn., Sept. 4, 2008.

Republican presidential nominee S... Photo Credit: Win McNamee/Getty Images
Despite his name recognition and political experience, McCain was defeated in the election by newcomer, Barack Obama, whom he conceded to on Nov. 4, 2008.

Republican presidential nominee S... Photo Credit: David McNew/Getty Images
McCain continued his career in the Senate, winning a sixth term in 2016. McCain supported Republican nominee Donald Trump though he found himself at odds with Trump and Trump disparaged his military service. Mc... Photo Credit: Ralph Freso/Getty Images
After being diagnosed with a brain tumor and having it removed, Sen. John McCain returned to the Senate to vote on the recent healthcare bill reform, July 25, 2017, in Washington, DC. Photo Credit: Win McNamee/Getty Images
Despite his battle with brain cancer, John McCain continued to fulfill his duties as senator, voting on important bills such as healthcare and changes to the tax code, and was honored with the Outstanding Civil... Photo Credit: J. Scott Applewhite/AP Photo

As he reflected on their friendship, Lieberman recounted a trip he took with McCain to a Syrian refugee camp in Turkey.

“As we got closer we realized they were ... actually cheering and chanting our name, and the one that they chanted most was McCain, of course, because he had really made so clear, and they had known it, that his sympathies were with both the opposition but also with the refugees,” Lieberman said.

“This Three Amigo business was more than three different kinds of guys who became friends and sort of had a great time traveling around the world,” he added. “I think we really tried to stand for something.”