Which 1848 treaty transferred California and much of the modern U.S. Southwest to the United States?

The Treaty of Guadalupe Hidalgo transferred California and much of the modern U.S. Southwest to the United States.

Signed on February 2, 1848, the treaty ended the Mexican–American War. Mexico recognized the Rio Grande as the boundary of Texas and ceded a vast region known as the Mexican Cession. That territory included present-day California, Nevada, Utah, and most of Arizona and New Mexico, along with portions of Colorado and Wyoming.

The United States paid Mexico $15 million and assumed certain claims by U.S. citizens against the Mexican government. The treaty also promised protections for Mexicans living in the ceded lands, including property and citizenship provisions, although implementation was uneven and contested.

The Gadsden Purchase of 1853 later added a smaller strip of land in present-day Arizona and New Mexico. The Adams–Onís Treaty, by contrast, transferred Florida from Spain to the United States in 1819.