The Mexican Cession of 1848 is a historical name in the United States for the region of the modern day southwestern United States that Mexico ceded to the U.S. in the Treaty of Guadalupe Hidalgo in 1848, but had not been part of the areas east of the Rio Grande which had been claimed by the Republic, though the Texas annexation resolution two years earlier had not specified Texas's southern and western boundary. 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 820,000 sq. miles, followed by the acquisition of Alaska (about 586,000 sq. miles).