The Mexican-American war officially began on May 11, 1846, when President James K. Polk asked Congress for a declaration of war against Mexico. However, tensions between the United States and Mexico began long before that.
Declaration of War
When Texas became the 28th state in the Union, the annexation was not well-received by Mexico. Texas had been independent of Mexico since 1836, but it was a slave state, and the Northern states were not sure they wanted them to be a part of the union.
Polk had offered to purchase the land to the west from Mexico, but the Mexican government had no interest in selling. Polk initially moved troops into the zone between the Nueces River and the Rio Grande.
According to History.com, the area was heavily disputed and was recognized as part of the state of Coahuila. The Mexican cavalry began border raids along the Rio Grande and threatened war in response.
The first attack was on April 25, 1846, when 2,000 Mexican cavalry members attacked a 70-man U.S. Army patrol. In the attack, 11 Army members died. Six more Americans died at the Siege of Fort Texas and the Battle of Palo Alto.
When Polk addressed Congress, he said Mexico had "invaded our territory and shed the blood of our fellow citizens on our own soil." Polk wanted to take over all of North America, from the Atlantic to the Pacific, in one grand expansion.
Col. Stephen W. Kearny and Commodore Robert F. Stockton led the battles into Mexico easily. However, Mexican General Antonio Lopez de Santa Anna double-crossed Polk and assisted Mexico.
The Battle of Buena Vista did not end well for Mexico, but Santa Anna still took over the Mexican presidency. The tide turned in favor of the U.S. in September of 1847.
Much of the war was fought on Mexican soil because of the initial battles and Polk's desire to expand. But, interestingly enough, Mexico never declared war on the United States.
During the war, 13,000 American soldiers died. However, the Department of Defense only has 1,733 killed in combat.
Twenty-one months later, the U.S. purchased Arizona, Colorado, Nevada, Utah, New Mexico, and California to the United States in the Treaty of Guadalupe Hildalgo on February 2, 1848, for $15 million.