We analyze how dierent rules for exiting an alliance (simple majority, unanimity or unanimity with side payments) will aect the formation of strategic alliances. We find that no alliance structure is contractually stable under the simple majority rule. Once unanimous consent is required, asymmetric alliance structures consisting of two alliances are contractually stable. In addition, the grand alliance which is the ecient structure is stable. Allowing for side payments to compensate former partners improves eciency. Finally, we show that dierent rules of exit may coexist in dierent alliances in the long run.