http://en.wikipedia.org/wiki/Mexican-American_War
The Mexican–American War was an armed military conflict between the United States and Mexico from 1846 to 1848 in the wake of the 1845 U.S. annexation of Texas. Mexico did not recognize the secession and subsequent military victory by Texas in 1836, and considered Texas a rebel province.
In the United States, the conflict is traditionally referred to simply as the Mexican War. In Mexico, terms for it include intervención norteamericana en México (North American intervention in Mexico), invasión estadounidense de México (U.S. invasion of Mexico), and guerra del 47 (war of '47).
In the United States, the war was a partisan issue. Most Whigs opposed it. Southern Democrats, animated by a popular belief in the Manifest Destiny, supported it. The regular U.S. army saw a 13% desertion rate (versus a 15% average annual rate in peacetime at the time[3]), the highest in U.S. history for a foreign war.[4] Anywhere up to 4,000 U.S. military members defected to the Mexican side.[4]
The most important consequence of the war for the United States was the Mexican Cession, in which the Mexican territories of Alta California and Santa Fé de Nuevo México were ceded to the United States under the terms of the Treaty of Guadalupe Hidalgo. In Mexico, the enormous loss of territory following the war encouraged its government to enact policies to colonize its northern territories as a hedge against further losses.