This article identifies two strategic problems a defender faces in deterrence. The resolve problem arises when the defender lacks the capacity to retaliate against immediate aggression; the commitment problem arises when the defender cannot credibly commit to retaliate against future aggression. We develop a dynamic model of deterrence showing that, counterintuitively, deterrence is strongest under a combination of strong present resolve and ambiguous future commitment. A defender that is both resolved and fully committed cannot deter when an aggressor is sufficiently determined to revise the status quo. Intertemporal flexibility facilitates deterrence by leaving open the possibility that the aggressor's prospects may improve over time while keeping that possibility limited. The theory thus explains why strong commitment to retaliate may trigger deterrence failure and why preserving ambiguity in future commitment can be strategically beneficial.