They were two years apart, these two Yale boys, these sons of privilege, and so the moment of truth came first for John Kerry (search), later for George W. Bush. Each faced the same life-changing question as did so many others of their generation: what to do about Vietnam.

Kerry, part of the class of 1966, signed on with the Navy late in 1965, then had months to ponder his decision before actually entering officer candidate school after graduation. The war, his decision, his doubts, all hung over him as he spoke at commencement the following June.

"What was an excess of isolationism has become an excess of interventionism," he told fellow students. He had to know his life was set on a course for Vietnam.

For Bush, a member of the class of 1968, his last year in college seemed to signal the end of a time of innocence.

"The gravity of history was beginning to descend in a horrifying and disruptive way," he wrote in his 1999 biography. "By the time the ball dropped in Times Square to welcome 1968, the situation in Vietnam had escalated from a conflict to a raging war. Every night the newscast included a body count."

Bush debated his options over Christmas break back home in Houston, took a pilot aptitude test after he got back to school in January, and chose the National Guard. He would fly planes like his father did in World War II, but he had to know the odds of going to Vietnam were low.

Nearly 40 years later, the choices made by these two young men are reverberating through the presidential campaign as part of a larger debate over patriotism, leadership, duty, character. Each man is defined in part by the path he chose, and by the level of commitment he demonstrated along the way.

"We are all hostage to decisions we made in the past," said Douglas Brinkley (search), a history professor at the University of New Orleans who has written a book about Kerry's war years. "The bottom line is Kerry went and Bush didn't and it's an uncomfortable fact for a president" who has so eagerly wrapped himself in the flag as commander in chief.

Yet Brinkley said the two-year age difference between Kerry and Bush is an important backdrop to the courses they set.

In 1965, when Kerry decided to enlist, students "still saw the world in black and white," Brinkley said, and "not serving wasn't really an option" for the son of a foreign service officer. "His big decision was which branch of the military to join," said Brinkley. "Did he want to go to Vietnam? No. But how could he live with himself if he finagled his way out of his duty?"

By the time Bush joined the guard in 1968, Brinkley said, the horrors of Vietnam were playing out nightly on television and sentiment against the war was hardening. "By 1968, smart kids weren't going. It became OK not to go. ... So Bush looked for a way not to go," he said.

"If he had been the class of '66, it may have been different for George W. Bush."

Bush spoke about his decision to serve in the National Guard in an interview Sunday on NBC's "Meet the Press," saying, "I put in my time, proudly so. I would be careful to not denigrate the Guard. It's fine to go after me, which I expect the other side will do. I wouldn't denigrate service to the Guard, though."

Wesley Clark, a retired four-star general, is part of the campaign debate over military service, too, as he tries to keep his Democratic presidential campaign alive.

Clark, who viewed the military as a path of opportunity for a bright, middle-class kid from Arkansas, graduated first in his class at West Point in 1966, then headed off to Oxford on a Rhodes scholarship. He wrapped up his studies there in two years rather than three so he could get to Vietnam quicker, and came home with a Silver Star for heroism.

Clark still has scars on his shoulder, hip and leg, and his right index finger was shortened by a bullet during a firefight with the Viet Cong.

Lately, the campaign discussion has repeatedly turned to the candidates' military histories, with all sides faulting the others for exploiting the issue.

Neither Kerry nor Clark have made a point of personally raising Bush's military record on the campaign trail, but both say when asked that legitimate questions have been raised about the Bush record.

"It's almost like an inverted time warp," said Stanley Renshon, a political scientist and psychoanalyst at the City University of New York. "The point of it is that, 'I'm a war hero and you're not.' And the implication is that because you're a war hero, that gives you a special standing to talk about war and strategy. But it doesn't follow."

Hardly a speech goes by in which Kerry, a decorated war hero, does not invoke Vietnam and its legacy. Vietnam buddies travel with his campaign entourage and appear in his ads. At an emotional appearance just before Kerry's huge victory in the Iowa caucuses, he was reunited with a serviceman whose life he had saved as the skipper of a river patrol boat.

Kerry was awarded the Bronze Star for that rescue, less than a month after earning a Silver Star for beaching his boat and jumping ashore to chase down and kill a guerrilla who had a rocket launcher pointed at the Americans. After being awarded three Purple Hearts for minor injuries, Kerry's request for reassignment stateside six months early was granted.

His campaign mantra, "Bring it on," evokes the same never-back-down approach he had evinced in battle.

"The entire heart and soul of John Kerry's personna is Vietnam," said Brinkley. "What happened to him is so seared into his being that it's almost like rings in an old redwood tree."

Kerry's fierce criticism of the war upon his return to America did not sit well with some veterans, and still doesn't.

As an anti-war leader, he asserted in testimony to Congress that U.S. soldiers had "raped, cut off ears, cut off heads ... randomly shot at civilians ... poisoned food stocks" and committed other atrocities he later acknowledged he did not witness.

Bush, for his part, harked back to his fighter-pilot training last year when he climbed into a flight suit and flew in a Navy jet to land on an aircraft carrier off the California coast. He emerged from the plane with the swagger of a top-gun pilot, cradling his helmet under his arm, and shouted, "Yes I flew it!" to those on deck.

Now, he is facing a new round of questions about his guard service on issues that first came up during the 2000 campaign:

— Whether family connections helped him get into the Texas Air National Guard when there were waiting lists for what was seen as an easy billet. Bush says no one in his family pulled strings and that he got in because others didn't want to commit to the almost two years of active duty required for fighter pilot training.

— Whether he showed up for duty while assigned to guard units in Alabama, where he worked on a political campaign in 1972. Military records show no evidence he reported for duty. "There may be no evidence, but I did report," Bush told NBC, adding, "Otherwise, I wouldn't have been honorably discharged."

— Why he was allowed to end guard duty about six months early to attend Harvard Business School. Bush said on NBC he had "worked it out with the military. And I'm just telling you, I did my duty."

Maurice Udell, one of Bush's flight instructors at Ellington Air Force Base in Texas, remembers Bush as a standout student. "I'd rank him in the top 5 percent," says Udell, now 73 and retired. He rejects the notion that Bush got preferential treatment or that there was anything improper about his time in Alabama or in going to Harvard before his six-year guard commitment had ended.

"I was really a tough instructor but I was fair with him," Udell said, remembering Bush for his excellent memory and standout sense of humor. "I'd give him hell about something and he'd pop a joke and get you laughing and just break up the whole situation."

Udell says Bush asked about a program under which National Guard pilots were assigned to Vietnam, but Udell told him he wasn't eligible because he was certified on the F-102, which the military was phasing out.