5.3 The Mexican–American War

Mexico had never accepted Texas independence, and it disputed the border: Texas claimed the Rio Grande, while Mexico said the border was the Nueces River, farther north. President Polk also wanted to buy California and New Mexico, but Mexico refused to negotiate.

In 1846 Polk sent General Zachary Taylor's army into the disputed land between the two rivers. When Mexican troops attacked an American patrol there in April 1846, Polk told Congress that Mexico had 'shed American blood upon the American soil'. Congress declared war in May 1846.

Many Whigs, abolitionists and some New Englanders opposed the war. They saw it as an unjust land grab and suspected a 'Slave Power' plot to add slave territory. Abraham Lincoln, then a young Whig congressman from Illinois, introduced the 'spot resolutions', demanding that Polk identify the exact spot where American blood had been shed and suggesting it was actually Mexican soil. Henry David Thoreau refused to pay a tax in protest (4.9).

Supporters, especially Democrats in the South and West, saw the war as Manifest Destiny in action.

American forces won nearly every major battle. Taylor won in northern Mexico, other forces took New Mexico and California, and General Winfield Scott landed at Veracruz and captured Mexico City in September 1847. About 13,000 Americans died, most of them from disease.

In the Treaty of Guadalupe Hidalgo (February 1848), Mexico accepted the Rio Grande border and gave up the Mexican Cession: California, Nevada and Utah, most of Arizona, and parts of New Mexico, Colorado and Wyoming. The United States paid $15 million and took over about $3.25 million in claims American citizens had against Mexico. Counting Texas, Mexico lost about half its territory.

The treaty promised Mexicans who stayed in the ceded lands US citizenship and protection of their property. In practice, many lost their land through legal challenges, fraud and violence, and Mexican Americans faced discrimination.

In 1846, just months into the war, Pennsylvania Democrat David Wilmot proposed that slavery be banned in any territory gained from Mexico. The Wilmot Proviso passed the House, where the more populous North had a majority, but failed in the Senate, where the sections were evenly balanced.

The vote split by section rather than by party: Northern Democrats and Whigs voted against Southern Democrats and Whigs. That was a warning that slavery could break the national parties apart.

Several positions emerged:

• Free soil: ban slavery from all the new territories (Wilmot Proviso; the Free-Soil Party, formed in 1848).
• Extend the Missouri Compromise line of 36°30′ to the Pacific.
• Popular sovereignty: let the settlers of each territory decide by vote (Lewis Cass, later Stephen Douglas).
• Slavery everywhere: Congress has no power to ban slavery in any territory, because enslaved people are property protected by the Constitution (John C. Calhoun).