U.S. Congress Declares War on Mexico

Context

The roots of the war lay in the 1836 Texas Revolution and Mexico’s refusal to recognize the independent Republic of Texas. When the United States annexed Texas in 1845, Mexico severed diplomatic relations and continued to claim the territory as its own. The boundary dispute centered on whether Texas ended at the Nueces River or extended south to the Rio Grande, a difference that placed valuable lands and ports in contention.

What Happened

President Polk, elected on an expansionist platform embracing Manifest Destiny, sought to acquire California and New Mexico as well. After a diplomatic mission to purchase the territories failed, he ordered General Zachary Taylor’s forces into the disputed region between the Nueces and the Rio Grande. On April 25, 1846, Mexican troops attacked a U.S. patrol in what became known as the Thornton Affair, killing or capturing several soldiers. Polk received word of the clash on May 9 and, two days later, sent Congress a message asserting that Mexico had invaded American soil and shed American blood.

Aftermath

Congress acted swiftly. On May 12 the Senate approved the war measure 40–2; the House followed the next day by a wide margin. The resulting statute authorized the president to raise up to 50,000 volunteers, organize militia units, and spend $10 million to prosecute the war. American forces under Taylor quickly won early victories at Palo Alto and Resaca de la Palma, while naval squadrons began operations along both coasts of Mexico.

Legacy

The Mexican-American War ended with the 1848 Treaty of Guadalupe Hidalgo, by which Mexico ceded roughly half its territory—including present-day California, Nevada, Utah, most of New Mexico and Arizona, and parts of Colorado, Wyoming, Kansas, and Oklahoma—for $15 million. The vast new lands intensified sectional conflict over slavery’s expansion, producing the Wilmot Proviso debates and accelerating the political crisis that led to the Civil War. Historians view the episode as a pivotal assertion of U.S. continental power and a classic case of aggressive territorial expansion justified by claims of defensive necessity.