African-Americans are six times as likely as white Americans to die at the hands of a murderer, and roughly seven times as likely to murder someone. Young black men are 15 times as likely to be murdered as young white men. This disparity is historic and pervasive, and cannot be accounted for by individual characteristics. Culture-of-violence and tail-of-the-distribution theories are also inadequate to explain the geographic and demographic pattern of the disparity. We argue that any satisfactory explanation must take into account the fact that murder can have a preemptive motive: people sometimes kill simply to avoid being killed. As a result, disputes can escalate dramatically in environments (endogenously) perceived to be dangerous, resulting in self-fulfilling expectations of violence for particular dyadic interactions, and significant racial disparities in rates of murder and victimization. Because of strategic complementarity, small differences in fundamentals can cause large differences in murder rates. Differences in the manner in which the criminal justice system treats murders with victims from different groups, and differences across groups in involvement in street vice, may be sufficient to explain the size and pattern of the racial disparity.