Washington (CNN) -- During his 2000 presidential run, a New Hampshire voter asked Republican Sen. John McCain who he might choose as secretary of defense.
McCain responded by tossing out the name of a dear old friend and colleague, the very man who would be nominated to that post by a Democrat and frequent McCain target 13 years later.
"There's a lot of people that could do that," he told voters at a January town hall in Nashua, New Hampshire. "One of 'em, I think, is Sen. Chuck Hagel."
Now Hagel has been nominated for that same job, by McCain's 2008 campaign rival, President Barack Obama. But when the Senate Armed Services Committee meets on Thursday to discuss the nomination, McCain will be one of Hagel's biggest skeptics.
"My biggest concern is his overall attitude about the United States, our role in the world -- particularly in the Middle East and whether we should reduce the Pentagon further -- but mainly his general overall world view," McCain told CNN's Wolf Blitzer about his former colleague.
To understand the depth of the fallout between the two men one needs to understand how close the senators once were.
McCain loaned his star power to Hagel's first Senate campaign in 1996, traveling to appear with him in Nebraska.
When Hagel was elected to the Senate, the two men shared a bond over their service in Vietnam. In 1997 they attended a ceremony marking the 15th anniversary of the Vietnam Veterans Memorial.
Speaking about his comrades in arms, McCain mentioned the "unique bond that exists between us of which I am eternally proud."
"When I see them on the floor of the Senate or somewhere in the Capitol, I feel a little lift," he said.
Hagel spoke of McCain's service, declaring "no one during this entire time ever gave more, or showed more courage, than John McCain."
Their friendship led to a close working relationship during the 2000 presidential campaign, when Hagel served as a loyal surrogate and campaign co-chair for McCain.
Then, Hagel said his friend felt he owed the country his service. "His life has been committed to something larger than he is," Hagel said after McCain suspended his campaign effort to make way for an ascendent George W. Bush. "He's a selfless person who's given much to his country."
Later that year, Hagel introduced McCain on the stage of the party's convention with a giant embrace.
Back in the Senate, the long-time friends and combat veterans both voiced early concerns about the handling of the Iraq war.
But their views diverged as McCain came to support President Bush's proposed troop surge in 2007, a military decision that drew Hagel's scorn. He famously called the proposal "the most dangerous foreign policy blunder in this country since Vietnam if it is carried out."
A source close to McCain said the friends just drifted apart as friends sometimes do, not because of policy differences. But even McCain acknowledged the change.
"I've noticed over the years that our views on the United States of America and what we should be doing in the world have diverged rather dramatically," he said earlier this month.
By McCain's 2008 presidential run, Hagel was more in line with McCain's anti-Iraq war opponent Barack Obama.
The two traveled together on a Middle East Senate trip Obama used to beef up his foreign policy chops, and Hagel was never seen on McCain's iconic Straight Talk Express campaign bus.
Hagel and McCain spoke after the former senator's nomination to the Defense Department. Afterward, McCain said he had concerns about Hagel's positions on Iran, Iraq and Afghanistan. But citing their long friendship, McCain said he would not oppose Hagel before the hearing.
"Sen. Hagel and I are, are old friends and we had a very frank and candid conversation and I'll be looking forward to the hearing and asking questions," McCain said on January 22. "I think that he should be given the opportunity of a hearing before any of us make a judgment."