Theories of alliance formation and war suggest that alliances influence the probability that a potential challenger will initiate a militarized interstate dispute. This is because alliances are expected to influence their members’ likelihood of intervening in a potential war. More specifically, defense pacts are expected to increase the likelihood of members joining the target of a war and offense pacts are expected to increase the likelihood of members joining the initiator of a war. However, there is no empirical evidence that demonstrates that these different alliance treaty obligations have these effects on states’ war intervention decisions. Therefore, in an analysis of all states’ war intervention behavior in 95 wars, I provide the first estimates of these different effects of alliance agreements. After controlling for a number of measured and unmeasured factors, I find that alliances have the hypothesized effects on states’ war intervention decisions. These findings provide a more comprehensive account of the effects of alliances on war intervention and offer additional support for arguments linking alliances to the onset of militarized interstate disputes.