The 1850s offered the last remotely feasible chance for the United States to steer clear of Civil War. Yet fundamental differences between North and South about slavery and the meaning of freedom caused political conflicts to erupt again and again throughout the decade as the country lurched toward secession and war.

With their grudging acceptance of the Compromise of 1850 and the election of Franklin Pierce as president in 1852, most Americans hoped that sectional strife and political upheaval had come to an end. Extremists in both North and South, abolitionists and secessionists,...