Why do states provide their allies with material support in the form of arms transfers in some cases but not others? I argue that arms transfers solve a critical problem arising from shaky alliance commitments. If a protege is worried that it will be abandoned by an ally in the future, it may initiate a preventive strike in the present to entrap its ally in war to capitalize on the benefits of joint military action. Thus, unstable alliances result in war because they give rise to a dynamic commitment problem. Arms transfers act as a substitute for an ally's direct support. As a result, arms reduce a protege's reliance, and soften the blow of expected abandonment. Thus, they eliminate the incentive for entrapment and result in peace. To illustrate this, I develop a formal model in which a defender, protege, and challenger interact repeatedly over time. When the defender's future commitment to the protege is certain, arms transfers are not necessary. However, when the defender's commitment is likely to wane, arms transfers may occur in equilibrium, as they prevent the protege from initiating a war of entrapment. This explanation provides insight into patterns of arms transfers that are not well explained by existing theory.