The Mexican War prompted a decade-long debate over the expansion of slavery into the newly acquired lands. This bitter struggle led to the Compromise of 1850, a complex legislative agreement that won little support either in the North or in the South and divided the Whig Party. As southern Whigs became Democrats and northern Whigs turned into Republicans or Know-Nothings, the parties split along sectional lines. The Kansas-Nebraska Act of 1854 began a downward spiral of political conflict that ended in the election of Republican Abraham Lincoln in 1860 and the secession of thirteen southern states. Chapter 13 details this breakdown of the political system.

In the long Civil War that followed, the military forces of the North and South were at first evenly matched. However, the North’s superior financial and industrial resources gradually gave it the advantage, as did Lincoln’s proclamation of freedom for slaves in 1863. Emancipation undermined European support for the secessionists and added thousands of African Americans to the northern armies. Union forces swept across the South and ended the war, which left a legacy of half-won freedom for blacks and decades of bitter animosity between northern and southern whites. The Civil War is the focus of Chapter 14.