This article uses data on prisoners incarcerated for misdemeanors in late-nineteenth-century U.S. cities to assess a three-part argument that asserts that threats to white dominance prompted efforts of social control directed against African Americans and foreign-born whites: (1) For African Americans, competition with whites for jobs instigated efforts by whites to enforce the racial barrier. (2) For the foreign-born, upward mobility became associated with white identity, which allowed those who “became white” to be seen as less threatening. We thus expect the threat from foreign-born whites to be highest where their concentration in poverty was greatest. (3) We suggest that violence against a given boundary raises the salience of group threat, so a positive relationship should exist between prior violence against a group and its level of incarceration for misdemeanors. Using panel analyses of cities from 1890 through 1910, we find supporting evidence for the first two arguments and partial support for the third.