Since the Vietnam War, U.S. policymakers have worried that the American public will support military operations only if the human costs of the war, as measured in combat casualties, are minimal. Although the public is rightly averse to suffering casualties, the level of popular sensitivity to U.S. military casualties depends critically on the context in which those losses occur. The public's tolerance for the human costs of war is primarily shaped by the intersection of two crucial factors: beliefs about the rightness or wrongness of the war, and beliefs about the war's likely success. The impact of each belief depends upon the other. Ultimately, however, beliefs about the likelihood of success matter most in determining the public's willingness to tolerate U.S. military deaths in combat. A reanalysis of publicly available polls and a detailed analysis of a series of polls designed by the authors to tap into public attitudes on casualties support this conclusion.