Mexican Cession
The Mexican Cession (Spanish: Cesión mexicana) is the territory that Mexico ceded to the United States in the Treaty of Guadalupe Hidalgo in 1848 after the Mexican–American War. It comprises the states of California, Texas, New Mexico, Utah, Nevada, and Arizona, and parts of Colorado, Oklahoma, Kansas, and Wyoming in the present-day Western United States. Consisting of roughly 529,000 square miles (1,370,000 km2), not including Texas, 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 ceded territory had not been claimed by the Republic of Texas following its de facto independence in the 1836 revolution. Texas had only claimed areas east of the Rio Grande. After annexation into the U.S. and admission as a state in 1845, Texas's southern and western boundaries with the Mexican state of Santa Fe de Nuevo México had not been defined by the U.S. Most of the ceded area was part of the Mexican province of Alta California (Upper California) while the southeastern strip east of the Rio Grande had been part of the state of Santa Fe de Nuevo México. Mexico had controlled the ceded territories with considerable local autonomy. After the Mexican War of Independence finished in 1821, the area had seen several revolts but the Mexican government sent few troops from central Mexico to suppress them. In 1846, U.S. President James K. Polk sent soldiers to occupy the disputed territory between the Nueces River and the Rio Grande, starting the Mexican–American War. At the war's outbreak, the U.S. conducted a naval landing on Mexico's northeastern coast on the Gulf of Mexico, and advanced into Nuevo México.
The northern boundary of the ceded territories was at the 42nd parallel north of latitude which was originally decided by the Adams–Onís Treaty of 1821 signed by the United States and the Kingdom of Spain in 1821, and was further ratified by the successor independent state of Mexico in 1831 in the Treaty of Limits. The eastern boundary of the Mexican Cession was the former old Texas Republic claim of additional territory to the west. The southern boundary was set by the war-ending peace Treaty of Guadalupe Hidalgo, which followed the Gila River and the original Mexican boundaries between Alta California to the north, and Baja California and the Mexican state of Sonora to the south.
The question of whether future Western states formed out of these 1848 Mexican Cession lands would permit the institution of slavery was a major American political issue in the lead-up to the American Civil War (1861–1865).