Expansion, Nationalism, and Sectionalism
Market revolution and nationalism
Canals, steamboats, railroads, and the telegraph tied regions together, while the cotton gin (1793) made slavery expand westward. Henry Clay's American System combined a protective tariff, a national bank, and internal improvements. John Marshall's Court strengthened federal power in McCulloch v. Maryland (1819) and Gibbons v. Ogden (1824). The Monroe Doctrine (1823) warned European powers against new colonization in the Americas, but it was a statement, not a treaty commitment.
Sectional crises
The Missouri Compromise (1820) paired Missouri's admission as a slave state with Maine's as free and drew the 36°30' line only across the Louisiana Purchase. The nullification crisis (1828-1833) pitted Calhoun's state sovereignty against Webster's nationalism; a compromise tariff and the Force Bill defused it. Indian removal produced the Trail of Tears despite Worcester v. Georgia (1832).
Expansion and breakdown
Manifest Destiny drove Texas annexation (1845), the Oregon settlement (1846), and the Mexican-American War, ending with the Treaty of Guadalupe Hidalgo (1848). The Wilmot Proviso failed, the Compromise of 1850 traded California's freedom for a harsh Fugitive Slave Act, and the Kansas-Nebraska Act (1854) repealed the 36°30' restriction, producing Bleeding Kansas and the Republican Party. Dred Scott (1857) declared the Missouri restriction unconstitutional.
Traps. Popular sovereignty means territorial voters decide, and each crisis changed the map of territory open to slavery.
Rule of thumb. Ask what each event did to the 36°30' line and to the free-slave balance in the Senate.
Worked example. Why did Kansas-Nebraska inflame the North? It introduced popular sovereignty in territory the Missouri Compromise had closed to slavery.