For nearly two years, from April 1846 to February 1848, the United States was at war with Mexico. North Americans now know the conflict as the Mexican (or Mexican-American) War. The war began over the issue of the Texas border, but it ended with a much larger transfer of territory. Under the Treaty of Guadalupe Hidalgo the United States gained the land that later became New Mexico, Utah, Nevada, Arizona, California, Texas, and western Colorado.

Background

In 1845 the Republic of Texas was set to enter the Union as the 28th state. Its border with Mexico would soon be the U.S. border. Texas claimed all the land north of the river called the Rio Grande. Mexico claimed that Texas extended only as far south as the Nueces River, about 100 miles (160 kilometers) northeast of the Rio Grande. In August, U.S. President James K. Polk sent troops to the region to show that the United States would defend the Texas claims. Still, he hoped to avoid war. In September he sent an agent to Mexico to settle the matter and also to offer to buy New Mexico and California for as much as 30 million dollars. Mexican officials, who did not want to part with any land, refused to see the diplomat.

In response, President Polk ordered U.S. troops to occupy the disputed area between the Nueces River and the Rio Grande. In late April 1846 Mexican troops crossed the Rio Grande, attacking U.S. soldiers. Sixteen men were killed or injured in the assault. Two weeks later Congress approved a declaration of war on Mexico.

Both Congress and the American people were divided in their opinion about the war. Those near the region approved of the war, since they hoped the United States could secure Texas. But others saw Polk as greedy and land-grabbing. Those who opposed slavery also tended to oppose the war since they feared that if Texas were given statehood, it would be as a slave state (where slavery was permitted).

The War

The United States fought the war on several fronts. A force led by Gen. Zachary Taylor was sent across the Rio Grande and fought several battles. After capturing the important city of Monterrey, Taylor defeated a major Mexican force at the battle of Buena Vista in February 1847.

Polk sent another army, led by Gen. Winfield Scott, across the Gulf of Mexico. In March 1847 Scott captured the important gulf seaport of Veracruz and began his march inland to Mexico City, the national capital. After an unbroken string of victories, Scott led his troops into Mexico City on September 14, 1847.

A third force under Col. Stephen Kearny went west into New Mexico and California. He encountered only light opposition. In California he was joined by Comdr. Robert F. Stockton and Lieut. Col. John C. Frémont. In January 1847 the last areas were brought under U.S. control.

Results of the war

On February 2, 1848, representatives from both sides signed the Treaty of Guadalupe Hidalgo. The United States gained possession of more than 500,000 square miles (1,300,000 square kilometers) of Mexican territory, from the Rio Grande west to the Pacific Ocean. The present-day states of the southwestern United States were eventually carved out of the vast region. In exchange, Mexico received 15 million dollars.

General Zachary Taylor's status as a war hero took him all the way to the White House by 1849. He and the Congress then had to deal with the question of the extension of slavery into the new areas. Slavery was permitted in Texas, but Congress passed the Compromise of 1850 to prohibit it in California. Other territories were allowed to decide the issue by vote.