Collective violence bracketed the Civil War and was important both to events leading up to the war and to its results.

In an upsurge of rioting in the mid-1830s, proslavery mobs predominated. Northern and Southern rioters attacked African Americans and abolitionists, attempting to silence the latter, but differences in sectional mob patterns laid the foundation for the coming struggles. Northern mobs commonly attacked property, and authorities checked them if they grew brutal or murderous; most of the people killed were rioters. Southern riots usually aimed at persons and were often sadistic or deadly, especially if the victims had been labeled abolitionists or insurrectionists. Authorities more often supported than controlled Southern mobs, so rioting became a communally sanctioned system of terror that quelled internal questioning of slavery.

In the 1850s, Catholic immigration and slavery were the primary explosive issues that resulted in violence. The 1850 Fugitive Slave Law, which...