How does the potential for socialization affect states' abilities to reassure each other and mitigate the security dilemma? Rationalist scholarship has identified numerous mechanisms by which states can credibly signal benign intentions. Yet it omits the possibility that states' interactions might endogenously shape their identities and domestic structures, and thus alter their basic preferences for or against cooperative outcomes. We present a formal model of the security dilemma that allows the sender's preferences to change endogenously as a function of the receiver's actions. The model yields several key results. First, the possibility of socialization generates incentives for benign actors to risk initiating cooperation, and even sustain cooperation in response to noncooperative signals in the hope of positively socializing the sender. However, conflict can still occur between mutually benign states through novel mechanisms not captured by standard models. These findings carry important implications for recent debates surrounding the US “engagement” strategy toward China.