he Mexican Cession is the region in the modern-day southwestern United States that Mexico ceded to the U.S. in the Treaty of Guadalupe Hidalgo in 1848. This region had not been part of 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 the southern and western boundary of the new State of Texas. The Mexican Cession (529,000 sq. miles) was the third largest acquisition of territory in US history. The largest was the Louisiana Purchase, with some 827,000 sq. miles, followed by the acquisition of Alaska (about 586,000 sq. miles).
