Summary
Highlights
The Mexican-American War was a consequence of US westward expansion and the ideology of Manifest Destiny. In 1844, President James K. Polk, a staunch expansionist, aimed to annex Texas, settle the Oregon dispute with Great Britain, and purchase California from Mexico. Mexico, however, refused to sell California, still resentful over the loss of Texas, which had declared independence in 1836 after American migrants settled there.
After annexing Texas in 1845, Polk provoked war by sending General Zachary Taylor’s troops into the disputed territory between the Nueces River (Mexico's claimed border) and the Rio Grande (Texas's claimed border). When Mexican troops crossed the Rio Grande, Polk declared that American blood had been spilled on American soil and Congress, fueled by Manifest Destiny, declared war in 1846. This declaration faced opposition from anti-war Whigs, including Abraham Lincoln, who questioned the exact location of the alleged attack.
The war ended in 1848 with the Treaty of Guadalupe Hidalgo. This treaty had three major effects: 1) The Mexican Cession granted the U.S. over half of Mexico's territory, including disputed Texas lands, New Mexico, and California, for $15 million. This significantly expanded the U.S. to the Pacific Ocean. 2) The acquisition of this new land intensified the dispute over the expansion of slavery. The Wilmot Proviso, which aimed to ban slavery in all new territories, was voted down but sparked outrage in the South, where some sought to expand slavery throughout all of Mexico. 3) The newly acquired lands were already populated by Mexican and American Indian communities, leading to immediate conflicts. The Comanche nation, which had previously harassed Mexican settlements, now faced American military might. Also, the relationship between white American settlers (Anglos) and native Mexicans (Tejanos) in Texas grew tense. Despite the treaty promising U.S. citizenship to former Mexican citizens, states often restricted it to white Mexican men. Anglos also expelled Tejanos from their land and forced assimilation, reducing many Tejanos to agricultural laborers. Juan Cortina, a wealthy Tejano landowner, led armed resistance against American expansion in the Cortina Wars along the Rio Grande.
Ultimately, the Mexican-American War boosted U.S. military prestige but reignited fierce debates over the expansion of slavery, which would contribute significantly to the sectional tensions leading to the Civil War.