Despite longstanding traditions of tolerance, inclusion, and democracy in the USA, dissident citizens and social movements have experienced significant and sustained -although often subtle and difficult-to-observe -repression. Using mechanism-based social movement theory, I explore a range of twentieth-century episodes of contention, involving such groups as mid-century communists, the Black Panther Party, the American Indian Movement, and the modern-day Global Justice Movement. Cracking open the black box of state repression, I demonstrate how four interactive social mechanisms -Resource Depletion, Stigmatization, Divisive Disruption, and Intimidation -animate state repression. A fifth mechanism -Emulation -diffuses the effects of these four Mechanisms of Repression. First I delineate a typology of state actions that suppress dissent. Then I shift analytically from these ten actions to the Mechanisms of Repression, explaining how these mechanisms work. Drawing on scholarship from an array of fields, and pulling data from a variety of sources, I explain how the state has engaged in activity that -operating through social mechanisms -inhibits collective action, either through raising the costs or minimizing the benefits of mobilization.