Theories about political movements typically posit models of actor choice that contain untested static assumptions about context. Short‐run changes in these contexts-induced by rapid shifts in the properties of political institutions-can alter choices and actors' interests, rapidly transforming the political landscape. China's Red Guard Movement of 1966-68 is a case in point. A generation of scholarship has attributed its violent factionalism to the opposed interests of different status groups. New evidence about the origins of the movement in Beijing's universities indicates that to the contrary, factions emerged when activists in similar structural positions made opposed choices in ambiguous contexts. Activists subsequently mobilized to defend earlier choices, binding them to antagonistic factions. Rapid shifts in the contexts for political choice can alter prior connections between social position and interests, generating new motives and novel identities. Close attention to these contextual mechanisms can yield novel accounts of the nature and origins of political movements.