It is often said that the Vietnam War taught us that the American public is no longer willing to tolerate American casualties in U.S. wars and military operations. There are also two contradictory corollaries: one that the first deaths in a conflict will spark demands for immediate withdrawal, the other that casualties lead to an inexorable demand for escalation to victory. The truth is far more subtle and sensible. The simplest explanation consistent with the data is that public support for U.S. military operations and public tolerance for casualties are based upon a sensible weighing of benefits and costs that is influenced heavily by consensus (or its absence) among political leaders. When such agreement is missing, even low costs can erode public support for the intervention. In the end, most Americans do not want lives to be sacrificed for any but the most compelling and promising causes, and they rely on their leaders to illuminate just how compelling and promising these causes are.