We identify two strategic obstacles a defender faces when trying to deter an aggressor. The resolve problem arises when the defender lacks sufficient willingness to retaliate against aggression today, while the commitment problem arises when the defender cannot credibly commit to retaliate in the future. We develop a dynamic model of war to show that, counterintuitively, deterrence is strongest when the resolve problem is absent but the commitment problem is present. A defender who is both resolved and committed cannot stop aggression today if the aggressor is sufficiently dissatisfied with the status quo. A resolved defender can improve deterrence by weakening its commitment to future retaliation. The commitment problem prevents aggression today by reassuring the aggressor that his prospects may improve over time. The theory sheds light on why patrons sometimes publicly distance themselves from protégés, why alliance treaties often embed flexibility and term limits, and why military support alternates between fungible aid transfers and the deployment of immobile assets.