The U.S. fought against Mexico over Texas and California.
The war was fought in the name of “manifest destiny,” the belief that the United States should possess the entire continent from the Atlantic to the Pacific oceans.
After settling Texas and admitting it to the Union in 1845, the U.S. wished to negotiate a settlement deal with Mexico, which had recently won its independence from Spain. Mexico declined to negotiate and the U.S. decided to use force.
American troops under the command of Zachary Taylor moved toward the Rio Grande, which was to be the border with Mexico. They met resistance from the Mexicans, and the skirmish marked the war's first conflict.
U.S. troops moved west and claimed Sante Fe, New Mexico (1846) and California (1847).
Leadership in Mexico changed hands several times during the war and caused confusion within the military. American troops overtook Mexico's weakened army and declared victory.
The Treaty of Guadalupe Hidalgo was signed in 1848. Mexico gave up two-fifths of its territory and received $15 million from the U.S. for damages.