The empirical literature has found that interstate alliances are, with the exception of the nineteenth century after 1815, usually followed by war rather than by peace. This analysis tries to identify theoretically the characteristics of alliances that distinguish those that are followed by war from those that are followed by peace. It is argued that alliances that embody settlements of territorial disputes are most peaceful. Alliances consisting exclusively of major states or of states that have been successful in their last war are predicted to be war prone, while those that have the opposite characteristics are predicted to be followed by peace. An empirical analysis of the data shows that all of the above expectations are confirmed. The analysis concludes by using these characteristics to reexamine the classic Levy, 1981, study.