Reflected on the wall listing in the names of American soldiers who died in the Vietnam War, a veteran helps decorate the Christamas tree placed at the Vietnam War Memorial in Washington on December 23, 2008. (Kevin Lamarque/courtesy Reuters)

Thirty-six years ago today the last U.S. Marines left Saigon. By nightfall, North Vietnamese forces seized control of the city, and South Vietnam ceased to exist. Anyone who lived through America’s exit from Vietnam—or who has relived it by watching the History Channel—knows that it was chaotic, ugly, and humiliating.

The consensus at the time was that Vietnam had dealt a grievous blow to American foreign policy, one from which the country might not recover. Much of the argument for keeping troops in South Vietnam after the Tet Offensive in 1968 rested on a perceived need to protect America’s credibility. The assumption was that failure in Vietnam would cripple American foreign policy, not just in Asia but around the world.

The thing is, history went in a different direction. Within four years of the fall of Saigon, China was at war with Vietnam, and the Soviet Union had begun its calamitous occupation of Afghanistan. Ten years out it was “morning in America,” and the United States was the dominant power in Asia. Sixteen years out, the Soviet Union joined South Vietnam on the ash heap of history. We now have normal relations with the communist government of Vietnam. China buys the Treasury notes that help us run a trillion dollar deficit.

Vietnam, it turned out, hadn’t been a strategic or vital interest to the United States after all, even though policymakers and pundits had insisted otherwise for years. We wound down our presence in Vietnam grudgingly and slowly, and only after vehement protests at home that tore at our social fabric. When we did leave, however ignominiously, we found ourselves freed from a policy that had been economically, strategically, and politically draining. The global geopolitical reshuffling we feared turned out not to be so daunting after all. Indeed, we learned to use it to our advantage.

Nearly sixty thousand Americans died in Vietnam. Hundreds of thousands of Vietnamese did as well. Cambodia became a killing field in a horrifying example of what people in my business euphemistically call “collateral damage.”

One of the last Americans to die in Vietnam was Corporal Charles McMahon. He was a Marine from Woburn, Massachusetts. It’s a blue-collar town not too far from the historic battlefields at Lexington and Concord. Charles was killed in a rocket attack on Saigon’s Tan Son Nhut airport, less than twenty-four hours before the city fell. He was twenty-one.

I sometimes wonder what might have been in store for Corporal McMahon and tens of thousands of others if the United States had taken Sen. George Aiken’s (R-Vt.) advice back in 1966 to “Declare victory and get out.” (Aiken actually said something else, but sometimes it’s what people remember you saying that matters more.)

History, of course, doesn’t allow for do-overs. But it does offer lessons. I would take three from Vietnam. The first is that if a policy isn’t working and it is economically, strategically, and politically draining, change course. And the sooner the better. Pretending that things are going to get better when you know they won’t isn’t strategy. It’s wishful thinking.

The second is that credibility in international relations is easier to recover than you think. Circumstances change, sometimes quickly, and countries care more about what you can do for them now than what you did to someone else before.

The third lesson is to be prepared for the future to defy your expectations. It seldom sticks to the script we write for it.