Principle and prudence sometimes conflict, but they can also reinforce each other. Consider the way the virtues of prudence and moral revulsion against killing innocent civilians affected the evolution of a nuclear taboo. Harry Truman used the new atomic weapon to end World War II without losing sleep over it, but he rejected proposals of nuclear use in 1948 when America had a nuclear monopoly. He again refused to use nuclear weapons to break the Korean stalemate in 1950 even though the United States had overwhelming nuclear superiority. Prudence about expanding the war and maintaining the support of allies was part of his decision, but he also said he was appalled at the idea of killing so many children.

Dwight Eisenhower threatened to use nuclear weapons as a means of creating deterrence in the Cold War, but at several points he rejected military advice to actually use them. While prudence became an increasingly important part of the mix of virtues after the Soviet Union exploded its first atomic bomb in 1949, privately Eisenhower also invoked moral convictions in explaining his decisions to his advisors in the 1950s. Similarly, John Kennedy’s prudence in seeking a compromise to end the Cuban Missile Crisis is a stark moral contrast to Johnson’s risky advocacy of an air strike. But neither Johnson nor Nixon seriously considered using (as opposed to threatening) nuclear weapons to escape their Vietnam imbroglio. The moral consequences of these “nonevents” were enormous. Had these presidential choices gone the other way, the world would look very different today.