Leeds et al. (2000) report that military alliance commitments are honored in war around 75% of the time. We update and extend data on alliance reliability from 1816 to 2003. Our analysis reveals a lower compliance rate overall: 50%. We find a sharp disparity in alliance reliability before and after World War II. States honored their alliance commitments 66% of the time prior to 1945 but the compliance rate drops to 22% from 1945 to 2003. Moreover, the rates of fulfillment for defense pacts (41%) and nonaggression pacts (37%) are dramatically lower than offensive alliances (74%) and neutrality agreements (78%). These findings carry implications for the role of military alliances in world politics and highlight the need for more research to explain the differences that emerge before and after World War II.