THE UNITED STATES SPENT $111 billion (more than $600 billion in 2013 dollars) and sent 2.6 million men and women to Vietnam. Of those, 58,200 never returned, and 150,000 suffered serious injury. The war shattered consensus at home, increased presidential power at the expense of congressional authority and public accountability, weakened the economy, and contributed to the downfall of two presidents.

Even as Nixon and Kissinger took steps to ease Cold War tensions with the major Communist powers — the Soviet Union and China — they also acted vigorously throughout the third world to install or prop up anti-Communist governments. They embraced their predecessors’ commitment to South Vietnam as a necessary Cold War engagement: To do otherwise would threaten American credibility and make the United States appear weak. Defeat in Vietnam did not make the United States the “pitiful helpless giant” predicted by Nixon, but it did mark a relative decline of U.S. power and the impossibility of containment on a global scale.

One of the constraints on U.S. power was the tenacity of revolutionary movements determined to achieve national independence. Overestimating the effectiveness of American technological superiority, U.S. officials badly underestimated the sacrifices that the enemy was willing to make and failed to realize how easily the United States could be perceived as a colonial intruder. A second constraint on Eisenhower, Kennedy, Johnson, and Nixon was their resolve to avoid a major confrontation with the Soviet Union or China. For Johnson, who conducted the largest escalation of the war, caution was critical so as not to provoke direct intervention by the Communist superpowers. After China exploded its first atomic bomb in 1964, the potential heightened for the Vietnam conflict to escalate into worldwide disaster.

Third, in Vietnam the United States sought to prop up an extremely weak ally engaged in a civil war. The South Vietnamese government failed to win the support of its people, and the intense devastation the war brought to civilians only made things worse. Short of taking over the South Vietnamese government and military, the United States could do little to strengthen South Vietnam’s ability to resist communism.

Finally, domestic opposition to the war, which by 1968 had spread to mainstream America, constrained the options of Johnson and Nixon. As the war dragged on, with increasing American casualties and growing evidence of the damage being inflicted on innocent Vietnamese, more and more civilians wearied of the conflict. Even some who had fought in the war joined the peace movement, sending their military ribbons and bitter letters of protest to the White House. In 1973, Nixon and Kissinger bowed to the resoluteness of the enemy and the limitations of U.S. power. As the war wound down, passions surrounding it contributed to a rising conservative movement that would substantially alter the post–World War II political order.