Why do states provide their friends with material support in the form of arms transfers in some cases but not others? I argue that temporal considerations are important, with arms transfers solving a critical problem arising from shaky third party commitments. If a protege is worried that it will be abandoned by a friendly third party in the future, it may initiate a preventive strike in the present to capitalize on the benefits of direct military support. Thus, unstable security relationships result in war because they give rise to a dynamic commitment problem. Arms transfers act as a substitute for the expectation of direct military support. As a result, arms reduce a protege's reliance, and soften the blow of expected abandonment. Consequently, 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'e is stable, 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.