Even for rich folks, insurance premiums can be a burden. The insurance Americans purchased for Southeast Asia eventually broke the bank—or at least the willingness of Americans to continue to pay.
In one sense Vietnam was inevitable. By the 1960s the American national interest was being defined so globally that hardly a sparrow could fall anywhere on earth without the U.S. government wanting to know why, to know whether the sparrow had jumped or been pushed, and, if pushed, to know whether the pusher wore scarlet plumage. Somewhere or other, sooner or later, the United States was bound to find itself defending a regime so weak, corrupt, or unpopular—especially since the chief criterion for American support was opposition to communism, rather than the positive embrace of democracy—as to be indefensible at any reasonable cost. The country where this occurred happened to be Vietnam, but it might have been Cuba (actually, it was Cuba also, but Fidel Castro worked too fast and cleverly for the Eisenhower and Kennedy administrations) or Iraq (Iraq likewise, but again the revolution succeeded before the United States reacted) or the Philippines (which similarly faced a leftist insurgency but managed to hold on).
Beyond its own problems, South Vietnam revealed something fundamental about the Cold War definition of the American national interest. As the world's only full-service superpower (the Soviet Union possessed a first-rate military but its strength in other areas was vastly overrated, as time revealed), the United States was more or less free to define its national interest however it chose. But having once agreed upon a definition, Americans were constrained to defend that definition lest they lose face with friends and enemies. Credibility counted when American commitments outran American capabilities. By no stretch of anyone's imagination could the United States have defended simultaneously all the regimes it was pledged by the 1960s to defend; its resolve and success anywhere had implications for its prospects everywhere.
That was why Lyndon Johnson and Richard Nixon went to such lengths to prevent the communist conquest of South Vietnam, and why Americans took failure there so hard. They might reasonably have accounted Vietnam simply as someplace where local conditions could not support an incompetent regime; if the American approach to Vietnam had actually (rather than metaphorically) been an insurance policy, Vietnam would have been written off and Americans would have gone about their business.
But Vietnam was not merely business—certainly not to the families and friends of the 60,000 service men and women who lost their lives there. Americans had indulged the illusion they could secure half the planet against revolution. In their post-Vietnam disillusionment, many Americans wondered whether they could secure any of the planet against revolution, or whether they ought to try. "For too many years," explained Jimmy Carter, the first president elected after Vietnam, "we've been willing to adopt the flawed and erroneous principles and tactics of our adversaries, sometimes abandoning our own values for theirs. We've fought fire with fire, never thinking that fire is better quenched with water. This approach failed, with Vietnam being the best example of its intellectual and moral poverty. But through its failure we have now found our way back to our own principles and values."
Carter was certainly speaking for his own administration, but how many other Americans his "we" comprised was problematic. The reflexively anticommunist definition of the American national interest—the definition that had enjoyed consensus support since the early Cold War—had indeed been discredited in Vietnam, but a credible replacement had yet to appear. Nixon's candidate, détente, based on the provocative notion that capitalism and communism—even Chinese communism—could coexist, had spawned an entire school of opposition, called neoconservatism. Carter's human rights–based approach appealed to those appalled by the dirty linen that kept tumbling out of the Cold War hamper, but struck others as naively woolly-minded.
The only thing nearly all Americans could agree on was that the national interest dictated avoiding anything that looked or smelled like another Vietnam. Liberals interpreted this to mean not sending troops to prop up ugly autocracies abroad. Neoconservatives interpreted it to mean not sending troops unless the U.S. government and the American people were willing to follow through to victory. With the blades of the last helicopters from Saigon still whomp-whomping in American ears, the liberal and neoconservative conditions amounted to the same thing.