Texas won independence from Mexico in 1836. Mexico refused to recognize Texas' independence and threatened war if the United States annexed its former territory.
Meanwhile, the United States wanted to expand its landholdings and used the concept of Manifest Destiny to fight for Texas and other land.
On May 11, 1846, President James Polk announced to Congress that the Mexican army had invaded U.S. soil and had killed 16 American soldiers, in an event known as the Thornton Affair. Congress declared war on May 13 despite allegations that increased territory would be used to increase slaveholding states.
The Mexican-American War reached California, then a Mexican territory, two months after war was declared. The success of the Bear Flag Revolt and the Siege of Los Angeles indicated an easy victory for the American soldiers. However, intervention by the Californios delayed success by the Americans for seven months.
The newly reinstated President Santa Anna took command of his army in Mexico after a failed battle, which resulted in the deaths of 11 graduates of this military academy, led by this commander in northeastern Mexico. Rumors of upheaval in Mexico City turned Santa Anna's attention back to the capital. An American general took possession of Mexico City and paved the way for the end of the war.
With the capture of Mexico City and the defeat of its army, Mexico surrendered to the United States and began negotiations to end the war. Read about the six brave young heroes who fought for Mexico.
The Treaty of Guadalupe Hidalgo was signed on Feb. 2, 1848. The terms of the negotiation ceded the land in what is present-day California, Nevada, Colorado, Wyoming, Utah, New Mexico, Kansas, Arizona and Texas.