Mexican-American War Begins

Setting the scene

The Mexican-American War began on April 25, 1846, following the U.S. annexation of Texas in 1845. The annexation, which Mexico never recognized, was driven by American expansionism and the doctrine of Manifest Destiny. President James K. Polk, elected in 1844 on an expansionist platform, sought to acquire California and New Mexico from Mexico. He sent John Slidell to Mexico City in 1845 offering $25 million for California and to settle the Texas boundary at the Rio Grande (Mexico claimed it was at the Nueces River, 150 miles north). Mexico refused to receive Slidell.

What happened

Polk then ordered General Zachary Taylor to occupy disputed territory between the Nueces and Rio Grande. On April 25, 1846, Mexican troops attacked a U.S. patrol in the disputed zone, killing 11. Polk claimed 'American blood on American soil' and asked Congress for war, which declared war on May 13, 1846. The war was controversial—many Whigs, including Abraham Lincoln, opposed it as aggression to expand slavery, since Texas would enter as a slave state. Henry David Thoreau was jailed for refusing to pay taxes supporting the war, writing 'Civil Disobedience.'

Why it still matters

The war lasted until 1848, with American victories at Palo Alto, Monterrey, Buena Vista, and the capture of Mexico City by Winfield Scott. The Treaty of Guadalupe Hidalgo (1848) ended the war, with Mexico ceding 55% of its territory—present-day California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado, Wyoming, Kansas, and Oklahoma—for $15 million. The war fulfilled Manifest Destiny but intensified sectional conflict over slavery expansion, leading directly to the Civil War. It remains controversial in Mexico as a traumatic loss, and in the U.S. as an example of aggressive expansionism.

Background

US forces occupied Mexico City two years later.