5.3 The Mexican-American War

Manifest Destiny Fuels Expansion

Belief in Manifest Destiny — the idea that the U.S. was destined to expand across the continent — drove the annexation of Texas in 1845 and heightened tensions with Mexico over the disputed border. President James K. Polk sought to acquire California and the Southwest, provoking conflict along the Rio Grande/Nueces River boundary dispute.

The Mexican-American War (1846-1848)

Polk sent troops into disputed territory, leading Mexico to attack, giving Polk justification to declare war. The U.S. won decisively, and the Treaty of Guadalupe Hidalgo (1848) ceded California and the Southwest (Mexican Cession) to the U.S. for $15 million, while the Gadsden Purchase (1853) added more land.

Slavery Debate Reignited

New territory reopened the question of slavery's expansion. The Wilmot Proviso proposed banning slavery in lands from Mexico but failed to pass, deepening sectional divides. Debates over popular sovereignty versus free-soil principles intensified, foreshadowing the sectional crises leading toward the Civil War.

Key terms

Manifest Destiny

The belief that the U.S. was destined to expand across North America, justifying westward expansion and war with Mexico.

Treaty of Guadalupe Hidalgo (1848)

Ended the Mexican-American War; Mexico ceded California and the Southwest to the U.S. for $15 million.

Wilmot Proviso

Failed proposal to ban slavery in territory acquired from Mexico; deepened sectional tensions over slavery's expansion.

Mexican Cession

The vast territory (California, New Mexico, and surrounding land) ceded by Mexico to the U.S. after the war.

Rio Grande vs. Nueces River dispute

Border disagreement between the U.S. and Mexico over Texas's southern boundary, which helped trigger the war.

Try an AP-style question

President Polk used which event to justify war with Mexico?

1. Mexico banning U.S. trade
2. A Mexican attack on Washington
3. A clash in disputed Texas territory
4. Mexico's invasion of Louisiana