In this chapter, we examined four related themes: the ideology of Manifest Destiny and the westward movement of Americans in the 1840s, the impact of American traders and settlers on the Indian peoples of the Great Plains and California, the causes and consequences of the Mexican War (1846–1848), and the disintegration of the Second Party System during the 1850s.

We saw that the determination of Presidents John Tyler and James Polk to add territory and slave states to the Union pushed the United States into the Mexican War and into a new debate over the expansion of slavery. To resolve the resulting crisis, Henry Clay, Daniel Webster, and Stephen Douglas devised the Compromise of 1850. Their efforts were in vain: antislavery northerners defied the Fugitive Slave Act, and expansionist-minded southerners sought new slave states in the Caribbean. Ideology (the pursuit of absolutes) replaced politics (the art of compromise) as the ruling principle of American political life.

The Second Party System rapidly disintegrated. The Whig Party vanished, and two issue-oriented parties, the nativist American Party and the antislavery Republican Party, competed for its members. As the Republicans gained strength, the Democratic Party splintered into sectional factions over Bleeding Kansas and other slavery-related issues. The stage was set for Lincoln’s victory in the climactic election of 1860.