Topic 5.3 Notes – The Mexican–American War

How the Mexican-American War Happened

This war grew out of Manifest Destiny, the belief that the United States was meant to spread across North America. That idea mixed patriotism, economic ambition, and racial assumptions that treated Mexicans and Native peoples as obstacles to expansion.

That expansion is easier to picture on a map because the crisis over Texas turned into a much larger shift in control of the Southwest.

Mexican Cession, Texas annexation, and the Gadsden Purchase

After Mexico won independence from Spain in 1821, it controlled Texas, New Mexico, and California. Mexico let U.S. settlers move into Texas, but tensions rose fast over slavery, immigration, religion, and how much authority Mexico could enforce.

Texas Revolution

In 1835-1836, Texans rebelled against Mexico.

• Alamo: Mexican forces defeated the defenders, which became a rallying cry.
• Goliad: Mexican troops executed captured Texans, deepening anti-Mexican anger.
• San Jacinto: Sam Houston’s forces won a quick surprise victory.
• Santa Anna captured: Antonio López de Santa Anna, Mexico’s leader, was taken prisoner.

Mexico never fully accepted Texas independence. So when the U.S. annexed Texas in 1845, it created an immediate crisis.

President James K. Polk wanted more than Texas. He wanted:

• Texas securely annexed
• California and New Mexico
• Pacific ports that could expand trade with Asia

Polk sent John Slidell to negotiate, but Mexico refused even to receive him. That failure made war more likely.

The border dispute pushed things over the edge. The map highlights the key claim the United States made at the time. The U.S. said the Rio Grande was Texas’s border. Mexico claimed the Nueces River. The land between them was disputed, and Polk sent troops there. In April 1846, the Thornton Affair broke out when Mexican troops attacked an American patrol. Polk used this to tell Congress that Mexico had “shed American blood upon American soil.”

Lock in the causes as a set:

• Texas annexation
• border dispute
• Polk’s desire for California and New Mexico
• Manifest Destiny
• Pacific trade and economic motives
• racial and cultural assumptions

Support and Opposition to the War

The war divided Americans because it was never just about one skirmish on the border. It raised questions about presidential power, expansion, and slavery.

Supporters, mostly Democrats and expansionists, said the war defended national honor and fulfilled Manifest Destiny. Many volunteers enlisted.

Opponents, especially Whigs and antislavery Northerners, argued Polk had provoked it.

• Abraham Lincoln’s Spot Resolutions challenged Polk to name the exact “spot” where blood had been shed.
• Frederick Douglass called the war immoral and proslavery.
• Henry David Thoreau protested the war and slavery by refusing to pay a poll tax. That experience led to Civil Disobedience.

How the United States Won the War

The U.S. had major advantages: a stronger economy, better artillery, naval superiority, and a more stable government. The overall pattern matters more than memorizing every battle. The U.S. attacked from several directions, seized the North and West, then captured Mexico City.

• Zachary Taylor fought in northern Mexico and won Buena Vista.
• Stephen Kearny moved into New Mexico and toward California.
• In California, the Bear Flag Revolt, John C. Frémont, and U.S. naval support helped secure the region.
• Winfield Scott launched an amphibious landing at Veracruz and marched inland to Mexico City.

The war lasted 1846-1848.

A detail teachers love to ask about is that many later Civil War leaders served here, including Ulysses S. Grant, Robert E. Lee, Stonewall Jackson, George McClellan, and Jefferson Davis. Grant later said the war was unjust.

What the Treaty of Guadalupe Hidalgo Changed

The Treaty of Guadalupe Hidalgo ended the war in 1848.

The later Gadsden Purchase in 1853 added southern land for a possible railroad route.

The biggest political effect was slavery. The Wilmot Proviso tried to ban slavery in land taken from Mexico. It passed the House, failed in the Senate, and never became law, but it exposed growing sectional voting.

Key positions:

• Free-soil: no slavery in western territories
• Southern proslavery expansion: slavery could expand west
• Popular sovereignty: settlers decide
• Abolitionism: slavery should end entirely

The treaty promised citizenship and property rights to Mexicans who stayed, but those promises were often weakened. Tejanos, Californios, and Hispanos lost land through courts, taxes, fraud, squatting, and violence. The California Land Act of 1851 made many prove old land claims in U.S. courts.

This land was also home to Comanche, Apache, Navajo, Pueblo, and others. U.S. expansion brought forts, settlers, conflict, dispossession, and growing pressure toward reservation systems.