The Mexican Cession of 1848 is a historical name in the United States for the region of the present day southwestern United States that Mexico ceded to the U.S. in 1848, excluding the areas east of the Rio Grande, which had been claimed by the Republic of Texas, though the Texas Annexation resolution two years earlier had not specified Texas's southern and western boundary.The U.S. had taken actual control of the Mexican territories of Santa Fe de Nuevo México and Alta California in 1846 early in the Mexican-American War, and Mexico acknowledged the loss of Texas, New Mexico, and California in the Treaty of Guadalupe Hidalgo, which was signed on February 2, 1848, ratified by the U.S. Senate on March 10, 1848, and by the Mexican government on May 19, 1848.