A theory of collective violence must explain both why it is collective and why it is violent. Whereas my earlier work addresses the question of why collective violence is violent, here I apply and extend Donald Black's theory of partisanship to the question of why violence collectivizes. I propose in general that the collectivization of violence is a direct function of strong partisanship. Strong partisanship arises when third parties support one side against the other and are solidary among themselves. Such (...) support occurs when third parties are socially close to one side and remote from the other and when one side has more social status than the other. Third parties are solidary when they are intimate, culturally homogeneous, and interdependent. I focus in particular on lynching: Lynching is a joint function of strong partisanship toward the alleged victim and weak partisanship toward the alleged offender. Unequal strong partisanship appears in both classic lynchings and communal lynchings across societies and history. Where partisanship is weak or strong on both sides, lynching is unlikely to occur. Evidence includes patterns of lynching in various tribal societies, the American South, imperial China, and medieval Europe. (shrink)