The Wilmot Proviso was an unsuccessful 1846 proposal in the United States Congress to ban slavery in territory acquired from Mexico in the Mexican–American War. The conflict over the Wilmot Proviso was one of the major events leading to the American Civil War.

The Mexican Cession 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, though the Texas annexation resolution two years earlier had not specified the southern and western boundary of the new state of Texas. At 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 and the 586,000-square-mile (1,520,000 km2) Alaska Purchase.