Mexican Cession - 1848
President Polk wanted to expand the U.S. as far as it could go and wanted all the territory to the west that was owned by Mexico.
Polk baited the Mexican Army into attacking the Americans and started the Mexican-American War. The U.S. quickly won the territories of New Mexico to California but had a rougher time in Mexico. After many fierce battles and many losses on both sides, Mexico signed the Treaty of Guadalupe-Hidalgo which gave the U.S. all the southwestern states up to the Pacific Ocean. This was equal to about ½ of Mexico’s territory.