Wartime periods have frequently been treated as natural macroeconomic experiments, but the international pooled time series evidence presented here shows that the literature has over-emphasized the experience of the United States and the United Kingdom. Wars fought exclusively on foreign soil do have marginally higher real output growth than peacetime periods, but real growth during all other wars is sharply below peacetime levels. Evidence for foreign and domestic wars is consistent with monetarist, fiscalist, and mixed theories of wartime booms.