The Mexican Cession (Spanish: Cesión mexicana) is the region in the modern-day Western United States that Mexico previously controlled, then ceded to the United States in the Treaty of Guadalupe Hidalgo in 1848 after the Mexican–American War. This region had not been part of the areas east of the Rio Grande that had been claimed by the Republic of Texas, which had been claiming independence since its Texas Revolution of 1836 and subsequent brief war for independence, followed afterwards a decade later by the American annexation and admitted statehood in 1845. It had not specified the southern and western boundary of the new state of Texas with New Mexico consisting of roughly 529,000 square miles (1,370,000 km2), not including any Texas lands, the Mexican Cession was the third-largest acquisition of territory in U.S. history, surpassed only by the 827,000-square-mile (2,140,000 km2) Louisiana Purchase of 1803 and the later 586,000-square-mile (1,520,000 km2) Alaska Purchase from Russia in 1867.
Most of the area had been considered the Mexican territory and province of Alta California (Upper California), while a southeastern strip on the river Rio Grande had been part of Santa Fe de Nuevo México, most of whose area and population were east of the Rio Grande on land that had been claimed by the new Republic of Texas since 1836, but never actually controlled or even approached (aside from the tragic Texan Santa Fe Expedition. Mexico controlled the territory later known as the Mexican Cession, with considerable local autonomy punctuated by several revolts and few troops sent from central Mexico and the capital of Mexico City, in the period from 1821–1822 after the Mexican War of Independence from the Kingdom of Spain up through to 1846 when U.S. military forces sent by 11th President James K. Polk (1795-1849, served 1845-1849) in a boundary dispute invaded the northeastern corner of Mexico between the Rio Bravo and the Rio Grande by land, plus another American naval landing on the nearby Gulf of Mexico western coast, plus seized control further to the far west of California on the Pacific Ocean coast with the landing of a U.S. naval squadron and two land expeditions across the continent from the east in Missouri and into the Mexican province of New Mexico at the outbreak of the Mexican–American War in 1846
The northern boundary of the territories ceded by Mexican to the U.S.A. was at the 42nd parallel north of latitude was originally set by the Adams–Onís Treaty of arbitration and border settlement signed two decades before by the United States and the Kingdom of Spain in 1821 after a long dispute and was further ratified by the successor state of an independent republic in Mexico in 1831 in the Treaty of Limits between them then. The eastern boundary of the Mexican Cession was the former old Texas Republic claim of additional western lands from the time of their Revolution of 1836 set at the Rio Grande and extending north to the headwaters of the Rio Grande, not corresponding to Mexican territorial boundaries. The southern boundary was set by the war-ending peace Treaty of Guadalupe Hidalgo, which used and followed the original Mexican boundaries between Alta California (to the north) and Baja California (Lower California) and the Mexican state of Sonora (to the south).
Until the American Civil War (1861-1865), the question of whether future Western states formed out of these 1848 Mexican Cession lands would or would not permit the institution of slavery in the newly acquired territories was a major American political issue in the following decade of the 1850s leading up as one of the causes of the later tragedy of Civil War.