John F. Kennedy was a fascinating figure, although no more fascinating than most of the other people that held the highest office in the United States. As a historian, what I find most interesting about him is the way he represents a lost consensus in American foreign policy, an era when the left wing and the right wing agreed on fundamental questions. Perhaps the questions became more complex, or perhaps one of the wings lost their way. That's not for me to say, at least not here. But consider what this liberal hero did - he started the American war in Vietnam, he sent the CIA to overthrow a foreign leader, and he faced down "the enemy" with a steel that we on the right sometimes wish his successors might have shared.

Kennedy was president at a time when the United States was transfixed by the Cold War, when the threat that the Soviet Union posed to the western world seemed obvious, and when the American left was particularly keen to demonstrate to voters that it understood this. These three conditions converged to produce Kennedy's foreign policy, and the consequences of his foreign policy eventually led to the reversal of them all. They all died in the jungles of Vietnam.

I think I write a generally accepted truth when I say that in our collective memory the 1950s were a decade of restraint and the 1960s were a decade of liberation. The '60s call to mind the pill, civil rights, and student rebellion. Kennedy, his life tragically cut short, became a symbol of this decade of liberation - but what isn't often appreciated is how he liberated American security and foreign policy from its contraints, with consequences that many Kennedy fans would be less than willing to praise.

Kennedy came after Dwight Eisenhower, a crusty old conservative whose belief in avoiding budget deficits was so great that he took the rare step in American history of attempting to limit the goals he set for the country because he was aware of its limited means. His approach to the Cold War was simple - tell the Commies that if they didn't watch themselves, they were gonna get nuked. Unfortunately, what he possessed in simplicity he lacked in credibility; it was difficult for anyone to believe that the United States would resort to nuclear war to solve every little dispute, but Eisenhower was unwilling to fund every facet of military power to allow for less dramatic means of response. He left office warning of the danger of allowing a military-industrial complex to emerge. Kennedy, it would transpire, was more willing to spend money on the military and avoid the cheap but suspect tactic of relying on mutually assured destruction.

This was an argument over how best to wage the Cold War and face down the Soviets, not the importance of doing so. And when Kennedy got into power, his fascination with ingenious ways of taking on Communism was legendary - no less so than in his love of counterinsurgency, an American tradition which lives on in Afghanistan today. Thus emerged the paradox of a president who believed equally in both the Green Berets and the Peace Corps.

Kennedy didn't realize it at first, but his main foreign policy problem was in Vietnam. Luckily, he already had an idea for dealing with it - counterinsurgency. It couldn't be more different from a nuclear war - American soldiers on the ground, among the native people, working against Viet Cong insurgents to fight them on their level and to deliver the benefits of American democracy and social progress. Kennedy wasn't above being taken in by the James Bond mystique, either - a demonstration for him of the prowess of the Green Berets at Fort Bragg, Carolina involved one plucky soldier taking off on a jet pack. Whether the equipment was ever used in combat is unclear - what is clear is Kennedy's belief in the ability of American technology and power.

The application of Kennedy's confidence to Vietnam would have been his undoing, had he lived long enough - and as it was, it proved the undoing of his successors. Kennedy tried to put the United States in the guerrilla warfare business so that it could win the Cold War in Asia, and he had his reasons - a belief that China and the Soviet Union were trying to expand their influence there, and a mortal fear that the loss of Vietnam to Communism would wound him domestically. Democrats were haunted in those years by the loss of China to Communism in 1949, and the charge that Harry Truman could have prevented it - Kennedy didn't want to have to answer for the loss of Vietnam. But he also sincerely believed in the importance of defending it.

But Vietnam spiralled down into violence and desperation, and Kennedy was dead before he could address it. His successor, Lyndon Johnson, was intellectually overawed by Kennedy and kept the latter's advisors always at his side - as the situation in Vietnam got worse, they kept committing more American troops to try to reverse the tide. They never abandoned Kennedy's belief that the United States could profoundly affect the course of a foreign war, even one which had such obvious social and political roots.

But gradually the country changed around them. As tens of thousands of Americans and millions of Vietnamese died, the American public began to question whether they could ever win, and whether it would be worth the cost if they did. The enormous sacrifices made by the GIs and the suffering of the Vietnamese were a very real demonstration of what a belief in America's role in policing the world meant. A decade of opportunity for the United States to change the world became a decade of disappointment, and those on the left rebelled against the Kennedy model. They would rarely trust American power to do good again. Sometimes, they would even forget what their hero taught - and what he did.