In May of 1846, U.S. President James K. Polk ordered the invasion of Mexico by U.S. troops, beginning the Mexican War. Three months later, General Stephen Watts Kearney led a victorious U.S. Army unopposed across northern New Mexico and into Santa Fe. Kearney’s conquest was formalized by the Treaty of Guadalupe Hidalgo, under which the U.S. paid Mexico 15 million dollars for New Mexico, Arizona and California. New Mexico became a territory of the U.S. in 1850.