The U.S.-Mexico War

The Dive

Texas had declared independence from Mexico in 1836, but when the U.S. annexed it in 1845, tensions reignited. Mexico viewed Texas as its territory. The U.S., under expansionist President James K. Polk, wanted more—including California and New Mexico.

Polk offered to buy the land. Mexico said no. So Polk sent troops to a disputed zone between the Nueces and Rio Grande Rivers—land both nations claimed. In April 1846, Mexican and U.S. troops clashed, and Polk called it an invasion on American soil.

On May 13, 1846, Congress declared war. Some saw it as defense. Others, like Frederick Douglass and Congressman Joshua Giddings, saw a war of conquest. The goal: land—and the question of whether slavery would spread across it.

The U.S. military quickly overpowered Mexican forces. Generals Zachary Taylor and Winfield Scott led campaigns that culminated in the capture of Mexico City in 1847. Meanwhile, antiwar sentiment grew, especially among northern abolitionists and working-class laborers.

The Treaty of Guadalupe Hidalgo ended the war in 1848. The U.S. paid $15 million for more than 500,000 square miles of Mexican territory—what’s now California, Arizona, New Mexico, Nevada, Utah, and parts of Colorado and Wyoming.

The war more than doubled the size of the U.S.—and intensified debates over slavery’s expansion. The Wilmot Proviso, proposed during the war, tried (and failed) to ban slavery in the new territories. The divisions it sparked would help ignite the Civil War a decade later.

For Mexico, the war was a national tragedy. For the U.S., it was a geopolitical jackpot—and a moral crisis. Its legacy lives on in the very shape of the border and in the memories of conquest, resistance, and the cost of ambition disguised as destiny.

Why It Matters

The U.S.-Mexico War wasn’t just about territory—it was about power, identity, and ambition. It shaped our map, but it also revealed the fault lines of race, empire, and slavery that still crack our politics today. Understanding this war helps us understand the border—and the border debates—that define the present.