Was the Mexican–American War fought for slavery?

The short answer

• Land hunger and an ideology of destiny. The brown arrow follows the Oregon Trail, traveled by thousands in the 1840s. As settlers moved west, they looked to Washington to secure the land they entered. Expansionists gave the movement a larger meaning through Manifest Destiny: Providence had supposedly assigned the continent to the United States. Pacific ports promised trade with China, while racial ideas encouraged the belief that Mexicans and Native peoples should give way.
• Polk’s determination to have California. Polk took office in 1845 determined to acquire California, whose harbors at San Francisco and San Diego offered access to the Pacific. He first sent John Slidell to Mexico City, authorized by most accounts to offer up to $25 million. Mexico refused to receive him. Polk then sent Taylor’s army to the Rio Grande, into land held by Mexico and claimed by Texas. An attempt to buy territory had given way to an armed standoff.
• A Mexican state at its weakest. Mexico had little room to respond to Polk. Its treasury was empty, generals made and unmade governments, and Comanche and Kiowa raids had devastated northern states. A leader who discussed selling land risked overthrow: Paredes toppled Herrera after Herrera showed willingness to talk. Mexico was poorly placed both to deter the United States and to bargain with it.
• Texas and the politics of slavery. The argument over Texas was also an argument over slavery. In 1844 Calhoun told the British minister that annexation would protect it from British abolitionism; the Senate rejected the treaty that June partly for that reason. Texas entered as a slave state by joint resolution in 1845. Northern opponents saw the “slave power,” their name for slaveholders’ supposed control of federal government, extending its reach. When war brought the prospect of still more territory, that suspicion followed it into Congress.

What it changed

Free soil becomes a party. Opposition to slavery’s expansion could unite Northerners who were not prepared to challenge slavery where it already existed. Their cause was free soil: western land reserved for free labor. In 1848 the Free Soil Party nominated former president Martin Van Buren and won about ten percent of the popular vote. Many of its leaders and its slogan, “Free Soil, Free Speech, Free Labor, and Free Men,” passed into the Republican Party in the 1850s. Lincoln’s 1860 platform carried forward the principle of the Wilmot Proviso.

The Fugitive Slave Act radicalizes the North. The Fugitive Slave Act was meant to reassure the South, but its enforcement brought the power of slavery into Northern towns. Accused fugitives received no jury trial, and federal commissioners were paid $10 for sending a person south and $5 for setting one free. Crowds in Boston and elsewhere tried to rescue captured people. Harriet Beecher Stowe wrote Uncle Tom’s Cabin in response. A concession to slaveholders became a source of Northern resistance.

Popular sovereignty carries the quarrel to Kansas. Why not let settlers decide whether to permit slavery? Popular sovereignty offered that democratic answer, and Stephen Douglas applied it to Kansas and Nebraska in 1854. His legislation repealed the Missouri Compromise’s 1820 line at 36°30′ between slave and free territory. On the continental map, hatching spreads across the plains in June 1854. The decision itself became the object of fighting: Kansas turned into a small civil war, carrying the quarrel from Congress to a frontier where no one could arbitrate it.

Mexico trains the generals of 1861. Grant, Lee, Jackson, Longstreet, McClellan, and Jefferson Davis served in Mexico before becoming commanders in 1861. They learned their trade in the same war and knew one another’s habits. Grant later wrote that the Southern rebellion “was largely the outgrowth of the Mexican war,” and saw the Civil War as punishment for that conquest. The earlier war had trained men who would fight over the question its victory reopened.