The Mexican-American War

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The Mexican-American War

War broke out against Mexico in May 1846. The United States Army was small, with only 8,000 soldiers, but 60,000 volunteers soon joined. The American Navy controlled the seas, and morale was high on the American side. The war quickly became a decisive victory for the United States.

President James K. Polk directed the war from Washington, D.C. He ordered a four-part attack into Mexico’s heartland. John Fremont and Stephen Kearny were sent to take control of California and New Mexico. Fremont led a group of Californians to declare independence as the "Bear Flag Republic." Although not taken seriously, Fremont and his followers captured the Mexican Presidio in Monterey. By 1847, California was secure.

Kearny led his troops into Santa Fe in 1846. The governor of New Mexico fled, and the city was captured without any fighting.

Attacks in Mexico
The attack on central Mexico was led by two other commanders. Zachary Taylor crossed the Rio Grande with his troops and fought against Santa Anna's army. Taylor was successful as he advanced toward the center of Mexico.

Winfield Scott led his troops to Mexico City, the capital. By the time they arrived, the Mexican government was forced to negotiate for peace.

Debates at Home
Not everyone in the United States supported the war. In the North, the Whigs criticized President Polk, saying his actions were misleading and possibly unconstitutional.

Abolitionists worried that new lands won from Mexico would be used to expand slavery. Antiwar sentiment was especially strong in New England. Writer Henry David Thoreau even went to jail for refusing to pay taxes that would fund the war.

The Treaty of Guadalupe Hidalgo
The Mexican-American War ended with the signing of the Treaty of Guadalupe Hidalgo in 1848. Through this treaty, the United States gained the disputed Texan territory, as well as the territories of New Mexico and California.

In return, the Mexican government received $15 million. Mexican citizens living in the new U.S. territories could choose to move to Mexico or stay where they were. Those who stayed were promised U.S. citizenship and protection of their property rights.

The United States Army had achieved a grand victory, but the new territories brought new challenges.

First page of the original Treaty of Guadalupe Hidalgo

The Debate Over Slavery Grows
Although the treaty expanded the United States, it also created more conflict over slavery. Many Americans wondered if slavery would be allowed in the new territories of California, New Mexico, and Texas.

Northerners who opposed slavery strongly argued against expanding it into the new lands. Southerners, however, believed they had the right to bring enslaved people into the territories. This disagreement caused heated debates across the nation.

One of these debates was over the Wilmot Proviso. This proposal aimed to ban slavery in all land gained from Mexico. While it passed in the House of Representatives, it was blocked in the Senate, where Southern states had more power. The failure of the Wilmot Proviso showed how divided the country was over slavery.

A Nation Divided
The lands gained through the Treaty of Guadalupe Hidalgo gave Americans new opportunities, but they also deepened divisions between the North and South. These debates over slavery would continue and eventually lead to even greater conflicts in the years to come.