Abstract: A key feature of military alliances is that their implementation is not instantaneous. Prospective alliance members decide to form an alliance and then factors beyond their control slow formal implementation. This dynamic alliance formation process has implications for the relationship between alliances and war. We develop a formal model of crisis bargaining in which alliance implementation unfolds over time. We find that alliances may cause war by inducing a commitment problem. Delay in implementation creates a window of opportunity in which the target of an alliance may strike to block the alliance. This points to a key conceptual innovation that emerges from our analysis: the decision to implement an alliance represents a rational gamble. While

some alliances may lead to war, the most dangerous alliances will often result in peace because implementation occurs quickly, precluding a preventive strike. Prospective allies will join alliances in spite of the risks. The analysis provides a unified theoretical framework to reconcile conflicting empirical findings on the relationship between alliances and war.