This article views death in battle as an opportunity cost whose size is determined by the number of years a rebel would have lived as a civilian. As civilian life expectancy declines, this opportunity cost does too, increasing the probability of rebellion. This theory is tested with a tragic natural experiment: the HIV/AIDS epidemic in sub-Saharan Africa. Using male circumcision rates as an instrument for life expectancy, the analysis shows that a one-year increase in life expectancy decreases the probability of civil war by 2.6 percentage points. This supports the theory that opportunity costs are important determinants of conflict onset and that nonpecuniary opportunity costs should be taken into account. This article concludes by noting that cost–benefit analyses of public health interventions should include decreases in the probability of civil war, and the attendant benefits in terms of lives saved and material damage prevented, in their calculations.