It was December 1968, and the United States was in turmoil. The number of U.S. troops in Vietnam had reached a peak of half a million. Antiwar protests were paralyzing American campuses after the assassinations of Martin Luther King Jr. and Robert F. Kennedy.

In Vietnam, a gangly U.S. Navy lieutenant named John F. Kerry steered his Swift boat up a river in the Mekong Delta, deep into enemy territory, witnessing firsthand a war he would soon conclude was no longer winnable. More than 8,000 miles away, in Valdosta, Ga., another son of privilege, George W. Bush, took off in a Cessna T-41 trainer, choosing to fulfill his wartime obligations stateside in the Texas Air National Guard.

Choices the two men made more than 31/2 decades ago have cast a long shadow over the current presidential campaign, helping to define the candidates to voters but also exposing them to harsh personal attacks. Bush has faced charges that he used family connections to dodge combat duty in Vietnam, while Kerry has been accused of betrayal for leading antiwar demonstrations after returning to the United States.

For both, the turbulent year of 1968 was the starting point for personal journeys that have been shrouded in myth and controversy. An examination of the way they grappled with the central foreign policy issue confronting their generation, based on their memories and interviews with people who knew them, provides insights into their political philosophies and preparation for the role of commander in chief.

The turmoil of Vietnam also affected the two vice presidential candidates. Richard B. Cheney began his ascent up the ladder of Republican Party politics in the fall of 1968, moving to Washington from Wisconsin to join the staff of a moderate Republican congressman after becoming ineligible for the draft. John Edwards's political epiphany came a few years later, in 1972, when he turned his back on his parents' Republican politics in part because of his disillusionment with President Richard M. Nixon's handling of the war.

Of the four men, only Kerry saw combat in Vietnam. George Q. Flynn, a retired professor who has studied Vietnam War-era conscription trends, said the majority of students avoided going to Vietnam, either by joining the National Guard or by getting draft deferments. Less than 10 percent of college students volunteered for active duty.

"There was nothing atypical about Bush joining the National Guard, which was considered a nice, safe haven from Vietnam," said Flynn, author of "The Draft, 1940-1973."

"It was Kerry who was the exception."

War and Peace at Yale

When President Lyndon B. Johnson escalated the Vietnam War in the fall of 1964, after accusing North Vietnam of firing on two U.S. destroyers in the Gulf of Tonkin, Kerry and Bush were attending Yale University. Kerry was a junior, Bush a freshman. Both would benefit from multiple student draft deferments.

Cheney had dropped out of Yale four years earlier because he could not keep up with the academic pace. Returning to Wyoming to attend community college, he received four student deferments, plus a fifth for "family hardship." Critical biographers have noted that Richard and Lynne Cheney had their first child in July 1966, nine months and two days after the Johnson administration expanded the draft to include married men without children.

The deferments kept Cheney out of the military until 1967, when he turned 26 and became ineligible for the draft. He would later insist that he complied with the conscription laws and would have been "happy to serve" had he been drafted. But as he told The Washington Post in 1989, "I had other priorities in the '60s than military service."

Kerry, by contrast, decided to volunteer for the Navy, inspired in part by the example of his political hero, John F. Kennedy, even though he had growing misgivings about the U.S. role in Vietnam. In a commencement day address to the Class of 1966, Kerry complained that the Johnson administration had moved from "an excess of isolationism" to "an excess of interventionism."

"We have not really lost the desire to serve," Kerry told classmates. "We question the very roots of what we are serving."

"Remember, we were the generation that heard John F. Kennedy say, 'Ask not what your country can do for you,' '' said Kerry's roommate, Dan Barbiero, who enlisted in the Marine Corps at the same time. "The doubts were not strong enough to fail to obey a call to arms."

Barbiero and others noted that there was a "huge difference" between Kerry's Class of '66 and Bush's Class of '68. While volunteering for active-duty military service was unusual in 1966, it was practically unheard of by 1968. A Bush roommate, Clay Johnson III, could think of only one close Yale acquaintance who served in Vietnam.

"By 1968, no one I knew would have considered going to Vietnam," said Lanny Davis, a Washington lawyer and former chairman of the Yale Daily News, who knew Bush and Kerry. "The place was awash with antiwar protests."

Bush, however, did not share those sentiments.

In an interview with The Post in 1999, he said he had no recollection of any antiwar activity on campus -- a remarkable statement, considering what was going on. The school's legendary chaplain, William Sloane Coffin, was a national leader of the antiwar movement and had been arrested for aiding draft resistance during Bush's senior year. Vice President Hubert H. Humphrey and Lady Bird Johnson were greeted by protesters when they visited Yale that year.

"George Bush had no political visibility whatsoever," said Gaddis Smith, professor emeritus of diplomatic history, who taught Bush and Kerry. "He was more like a student from the decade before, the mid-'50s, people who enjoyed their fraternity life."

According to Coffin, Bush "missed the great action and passion of his time." While Bush told The Post in 1999 that he generally supported the Johnson administration's position on the war, he did not feel strongly enough to speak out publicly.

"We were very apolitical," Clay Johnson said. "We didn't talk politics."

Some friends believe that Bush associated the antiwar movement at Yale with intellectual snobbery. "He had little sympathy for the antiwar people and their behavior and antics. They were pompous and pretentious," said roommate Robert J. Dieter. "They were 22-year-olds who thought they were going to run the world."

For Bush, deciding how to respond to the draft was a "practical" rather than a "moral" question, according to Clay Johnson, now deputy director for management of the president's Office of Management and Budget. Bush would later tell a reporter that he decided to join the National Guard because he was "not prepared to shoot my eardrum out with a shotgun" to get another deferment and was unwilling to move to Canada.

"That's irresponsible," he told classmate Robert R. Birge when Birge told him he was thinking of going to Canada to avoid the draft. "I'll respect you more if you just went to jail," Birge recalled Bush saying.

Bush completed his officer qualification tests on Jan. 19, 1968, scoring in the 25th percentile for pilot aptitude -- the lowest acceptable grade -- and the 95th percentile for "officer quality." Three weeks later, Kerry set sail for Vietnam on the USS Gridley. Between the two events, the Viet Cong launched the Tet offensive into the heart of Saigon, effectively putting an end to American hopes of winning the war.

Duty and Politics

Kerry took command of a 50-foot Swift boat on Nov. 17. Nine days later, Bush began pilot training at Moody Air Force Base in Valdosta.

Since Kerry volunteered for Swift boat duty, back in February, the mission had become much more dangerous. Instead of patrolling coastal waters, the boats were being sent upriver into hostile territory, to show the flag and disrupt enemy supply routes. During his four-month tour as a Swift boat skipper, Kerry would be awarded a Silver Star, a Bronze Star and three Purple Hearts for combat injuries, qualifying for an early return home.

Although Bush won positive evaluations from his flight instructors, his service during this time was pretty routine. The highlight came in the spring of 1969, when he was whisked off to Washington on a government plane for a date with Nixon's daughter Tricia. According to Bush biographer Bill Minutaglio, the date was arranged by his father, George H.W. Bush, then a Republican congressman from Houston, who was seeking Nixon's support to run for the Senate.

Cheney, meanwhile, had given up the idea of getting a doctorate in political science from the University of Wisconsin at Madison and moved to Washington on a congressional fellowship. He joined the staff of Rep. William Steiger (R-Wis.), who championed an all-volunteer army to replace the draft. "He came to Washington to study the political process," recalled Steiger's press secretary, Ted Cormaney. "But once he got here, he found that doing it was a lot more fun than studying it."

The Bush and Kerry biographies parallel each other again in 1971-1972, when each man began showing an interest in national politics. In Bush's case, this period coincided with a waning of interest in flying. After promising his Air National Guard superiors he intended to make flying "a lifetime pursuit," Bush was stripped of his wings in August 1972 after failing to take his required annual pilot's physical.

While the White House argues that the Air Force was phasing out the plane that Bush flew, military experts say it is nonetheless unusual for a certified military pilot to allow such a hard-earned credential to lapse.

Bush has never provided a full explanation for why did not take the physical, other than that he wanted to work on the Senate campaign of a family friend in Alabama, Winton M. Blount. According to Marion Knox, secretary to Bush's former commander, the late Col. Jerry B. Killian, Bush's decision upset his superiors because they had invested money and prestige in his flying career.

"Killian really liked Bush and had a lot of fun with him," Knox said. "But he was a by-the-book kind of guy and felt hurt when Bush skipped the physical."

While Bush was rethinking his commitment to the Air National Guard, and living out what he later referred to as his aimless "nomadic" years, Kerry made a name for himself in antiwar politics. He became a leading spokesman for Vietnam Veterans Against the War, but he resigned from the group in November 1971, after it began embracing more confrontational policies.

The following year, when Kerry ran for Congress from Lowell, Mass., the local newspaper blasted him as "a radical leftist war agitator" concerned only about his own political ambitions. He lost by a wide margin.

Edwards, meanwhile, had just turned 18, making him eligible for the draft, but no call-up cards were issued in 1972. Although Edwards grew up in a Republican family, he was so troubled by the war that he told his college roommate he could not vote for Nixon. But he could not support the Democratic presidential nominee, Sen. George McGovern (S.D.), either -- he was "too liberal."

"He was so torn he registered as an independent," said Bill Garner, a high school and college friend. While Edwards felt sympathy for the antiwar movement, he refused to take part in antiwar protests, according to Garner and Edwards's wife, Elizabeth.

"He was pushing ahead, trying to be the first person in his family to finish college, and didn't want to screw it up," said Elizabeth Edwards. "People were being arrested at those protests."

Commanders in Chief

People who knew Kerry at Yale and in Vietnam say that the experience of combat is likely to make him a more cautious commander in chief. They cite the case of Kennedy, whose experience in World War II as the skipper of a patrol boat in the Pacific made him hesitant to trust the assurances of the military brass.

"People who have had active combat experience and then reach high policy levels often turn out to be more cautious about starting wars than civilians," said Smith, Kerry's history professor at Yale. "What Kerry saw in Vietnam raised skepticism in his mind about the positions taken by the government in Washington."

"Vietnam had a tremendous impact on him," said Barbiero, who also served there. He believes that Kerry was thinking of the parallel between Vietnam and Iraq when he sought to draw a distinction between "the war and the warriors" in his first debate with President Bush.

The impact of the Vietnam War on Bush's thinking is much less clear. In his 1999 interview with The Post, he said he gradually became disillusioned with the war, particularly after Nixon's decision to bomb Cambodia in 1969. Bush said his disillusionment grew as he became aware that decisions were not being made "in the best interest of our military, but became political decisions." He added, "There was no clarity of purpose over time."

Some military historians dispute the claim that the lack of combat experience has made Bush more willing to commit U.S. troops to a foreign war than he might otherwise have been.

"There is something offensive about the idea that if you haven't been in combat, you are somehow insensitive to the horror of war," said Eliot A. Cohen, author of "Supreme Command: Soldiers, Statesmen and Leadership in Wartime."

"The war in Iraq was a difficult call which we are likely to be arguing about for a long time to come," Cohen said. "You can find Vietnam veterans on both sides of the argument."

Researcher Alice Crites contributed to this report.