By the time the Vietnam War ended in April of 1975, more than 3 million people (including 58,000 Americans) had been killed. The United States had entered the war in 1961 as the world’s preeminent power, still shining with the moral luster of its victory over totalitarianism in World War II, but left Vietnam with the American public sharply divided and American leaders uncertain what lay ahead in foreign policy.

Vietnam Syndrome

America had poured $120 billion into the war in an attempt to “contain” Communism, yet had failed abjectly. Ever since, American leaders have been afraid of getting stuck in “another Vietnam,” which would cost American lives and destroy American prestige. President Jimmy Carter, who took office in 1976, was the first U.S. President to be hamstrung by what some historians call “the Vietnam Effect” or “the Vietnam Syndrome,” which is a fear of using military power in almost any circumstance coupled with a defeatist attitude towards American involvement abroad. Carter’s botched attempt to rescue hostages from Iran and his failure to keep the Russians from invading Afghanistan were seen as further instances of American powerlessness.

No Quagmires

President Ronald Reagan’s “secret” war against the Nicaraguan Contras was secret precisely because he was afraid the American public would no longer support him after Vietnam, writes Harvard political scientist Richard Sobel in his book "The Impact of Public Opinion on U.S. Foreign Policy Since Vietnam." When President George H.W. Bush went to war against Saddam Hussein in the First Gulf War in 1991, he was careful to make sure other nations supported him and was able to say, at the war’s successful conclusion: “Thank God, we’ve kicked the Vietnam Syndrome, once and for all.” At the same time, showing how difficult it really is to erase Vietnam from the consciousness of American foreign policy-makers, he refused to give aid to Kurds rising up against Iraq and Hussein, saying that we did not want to find ourselves in a “quagmire” in Iraq.

An All-Volunteer Force

Another way the Vietnam War changed America’s approach to foreign policy was that the armed forces shifted to an all-volunteer army from a draft. Because of widespread sentiment against the military after the Vietnam War, the government could no longer trust the caliber of recruits it was getting from civilians pulled in by the draft, and had to go to a smaller volunteer force. This turned out to be a positive development, as the all-volunteer force, in a post-Cold War era of smaller wars, is more agile and responsive to government needs.

The Continuing Effect

Ultimately, however, the Vietnam War stopped the post-World War II era of aggressive and unquestioning U.S. involvement in foreign policy. President Barack Obama repeatedly invoked the Vietnam War in discussing why he wanted to wind down the war in Afghanistan within a timetable. It is likely that, in the absence of some clear-cut and major American military victory in the future, the Vietnam Syndrome will continue to impact the foreign policy of the United States.