Since the dawn of the Cold War, American policymakers and commentators have repeatedly insisted that the U.S. defend allies in one part of the world to show allies in others that America’s promises enjoy “credibility.” And again and again, the result has been to silence discussion of whether the country in question actually merits the expenditure of American money and the spilling of American blood.

In the immediate aftermath of World War II, George Kennan urged his superiors in the Truman administration to distinguish between those areas of the globe that were important enough to defend against Soviet advance and those that were not. But by the Korean War, Kennan’s more limited strategy was overtaken by Paul Nitze’s NSC-68, which insisted that, “any substantial further extension of the area under the domination of the Kremlin would raise the possibility that no coalition adequate to confront the Kremlin with greater strength could be assembled [anywhere else].” Whether the country under Soviet threat mattered in its own right was now irrelevant. Every country mattered because if the U.S. acquiesced to Soviet domination anywhere, it would lose credibility everywhere. “The effect,” writes John Lewis Gaddis, “was to vastly increase the number and variety of interests deemed relevant to [American] national security, and to blur distinctions between them.”

But the real disaster came in Vietnam. As a general rule, the men who led America into war did not see Vietnam itself as of great value. What haunted them was the fear that if America did not uphold its commitments there, it would demoralize America’s allies, and embolden the Soviets, in places that really mattered, like Central Europe. “Around the globe, from Berlin to Thailand,” declared Lyndon Johnson in April 1965, “are people whose well-being rests, in part, on the belief that they can count on us if they are attacked. To leave Vietnam to its fate would shake the confidence of all these people in the value of an American commitment and in the value of America’s word.” If the United States did not uphold its guarantees to Saigon, added Secretary of State Dean Rusk, its “guarantees with regard to Berlin would lose their credibility.”

Ironically, the very European leaders whose morale Johnson and Rusk feared undermining if America abandoned South Vietnam—men like British Prime Minister Harold Wilson and French President Charles de Gaulle—privately urged the U.S. not to escalate the war. In the end, after tens of thousands of Americans and millions of Vietnamese had died, the United States did abandon South Vietnam. And the world shrugged. Yes, communists racked up victories in some other corners of the developing world in the 1970s. But they lost ground in others. And in the heart of Europe, the place American policymakers really cared about, NATO held together and the Soviets stayed on their side of the Iron Curtain.