Do alliances really deter?

The scholarly literature is still divided on the relationship between defensive alliances and interstate conflict. While some scholars argue that defensive alliances can deter conflict, others posit that alliances accelerate its approach. This article seeks to make headway in the debate by using a research design that examines whether the recent formation of defensive alliances leads to increases or reductions in militarized disputes and war. We find that this relationship differs in the pre and post-nuclear era. In the prenuclear era, alliance formation is positively associated with both the initiation of militarized disputes and war onset. In the nuclear era, however, forming certain types of alliances reduces the likelihood of militarized dispute initiation, but has no effect on whether war occurs. This suggests assertions that defensive alliances will consistently deter conflict should be tempered and that alliance formation can sometimes undermine efforts to preserve peace.