How do countries’ conscription systems affect their ability to fight wars? Scholars have devoted significant attention to understanding how domestic political concerns influence military strategy, but we do not yet know how these concerns are shaped by military labor policies. We argue that conscription systems determine how the human costs of war are distributed throughout society, and in turn, the government’s tolerance for battlefield casualties in pursuit of victory. Using new data on every country’s conscription policy from 1800 to the present, we demonstrate that countries with selective conscription experience more casualties than those with universal conscription or volunteer militaries. To examine the mechanism we theorize, we then conduct an in-depth case study of the United States’ experience during the Vietnam War. Using a difference-in-differences design and new data on all American deaths in Vietnam, we show that changes in county death rates after the introduction of the lottery reflect electoral considerations.