"It's hard to recapture the horror that earlier generations of Americans felt about preventive war when it was still something that other countries did to the United States and not merely something Americans contemplate doing to others. They viewed it the way some Americans still view torture: as liberation from the moral restraints that human beings require. One of the things that frightened them most about the Nazis was that Hitler had dispensed with the concept of original sin. He had aimed to create a new class of infallible, god-like, humans who need not be encumbered by the fetters that bound lesser races. Totalitarianism, argued Arthur Schlesinger in The Vital Center, aimed 'to liquidate the tragic insights which gave man a sense of its limitations.' For Schlesinger, Reinhold Niebuhr, Walter Lippmann and other intellectuals who shaped America’s foreign policy debate in the early Cold War, acknowledging these limitations was part of what made America different. Because Americans recognized that they were fallible, fallen creatures, they did not grant themselves the illegitimate, corrupting power of preventive war."
Peter Beinart at The Atlantic looks at the American embrace of preventive war.