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The Mexican Cession (red) and the Gadsden Purchase (yellow)

The Mexican Cession is a historical name for the region of the present day southwestern United States that was ceded to the U.S. by Mexico in 1848 under the Treaty of Guadalupe Hidalgo following the Mexican-American War. The cession of this territory from Mexico was a condition for the end of the war, as United States troops occupied Mexico City, and Mexico risked being completely annexed by the U.S. The United States also paid $15,000,000 ($298,310,309 in 2005) for the land, which was half it had offered for the land prior to the war. Under great duress, Mexico accepted the offer. The land ceded by Mexico is 14.9% of the total area of the current United States territory.^[1]

For the 38 years between 1810 when Mexico declared its independence from Spain (or from 1821, when Mexican independence was secured), and 1848, the region had formed approximately 42.1% of the country of Mexico; prior to that, it had been a part of the Spanish colony of New Spain for some three centuries. Beginning in the early seventeenth century, a chain of Spanish missions and settlements extended into the New Mexico region, mostly following the course of the Rio Grande from the El Paso area to Santa Fe, which was a colonial capital under the Spanish, and which is now the capital city of the U.S. state of New Mexico. Spanish settlement and missionary work followed the course of the Colorado River northward from its mouth along the current border between California and Arizona. Beginning in the late eighteenth century, Spain had also built a system of fortresses and missions throughout Alta California (now southern half of the US state of California), from San Diego to San Francisco.

The region includes all of the present-day states of California, Nevada, and Utah, as well as portions of:

• Arizona excluding the region later annexed in the Gadsden Purchase
• Colorado west of the boundary of the former Republic of Texas
• New Mexico west of the Rio Grande and excluding the Gadsden Purchase
• Wyoming west of the former Republic of Texas boundary and south of the 42nd parallel.

The treaty also specified the Texas-Mexican border as being at the Rio Grande (Río Bravo del Norte). Previously the portion of Texas between the Nueces River and the Rio Grande had remained disputed throughout the existence of the Republic of Texas. The United States had already claimed the area as part of the Texas Annexation in 1845.

• State of Deseret
• Historic regions of the United States

1. ^ Table 1.1 Acquisition of the Public Domain 1781-1867