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The Mexican Cession of 1848 is a historical name in the United States for the region of the present day southwestern United Statesmarker that Mexicomarker ceded to the U.S. in 1848, excluding the areas east of the Rio Grandemarker, which had been claimed by the Republic of Texas, though the Texas Annexation resolution two years earlier had not specified Texas's southern and western boundary.

The U.S. had taken actual control of the Mexican territories of Santa Fe de Nuevo México and Alta Californiamarker in 1846 early in the Mexican-American War, and Mexico acknowledged the loss of Texas, New Mexico, and California in the Treaty of Guadalupe Hidalgo, which was signed on February 2, 1848, ratified by the U.S. Senate on March 10, 1848, and by the Mexican government on May 19, 1848.


A map of Mexico, 1835-1846 with separatist movements highlighted.

The cession of this territory from Mexico was a major goal of the war. California and New Mexico were captured soon after the start of the war and the last resistance there was subdued in January 1847, but Mexico would not accept the loss of territory. Therefore during 1847 United States troops invaded central Mexico and occupied the Mexican capital of Mexico Citymarker, but still no Mexican government was willing to ratify transfer of the northern territories to the U.S. It was uncertain whether any treaty could be reached. There was even an All of Mexico Movement proposing complete annexation of Mexico among Eastern Democrats, opposed by Southerners like John Calhoun who wanted additional territory for white Southerners and their black slaves but not the large population of central Mexico. Eventually Nicholas Trist negotiated the Treaty of Guadalupe Hidalgo ceding only California and New Mexico in early 1848 after President Polk had already attempted to recall him from Mexico as a failure. The U.S. Senate approved the treaty, rejecting amendments from Jefferson Davis to also annex most of northeastern Mexico and by Daniel Webster to not even take California and New Mexico.

The United States also paid $15,000,000 ($298,310,309 in 2005) for the land, and agreed to assume $3.25 million in debts to US citizens. The land ceded by Mexico is 14.9% of the total area of the current United States territory. 1.36 million km² (525000 square miles, or 55% of its pre-war territory) was lost by Mexico.

As of July 1, 2007, the states of California, Nevada, Utah, Arizona and New Mexico had a total population of 50,072,597 out of 305,986,357 nationwide, or 16.4% of the US population.

For the years between 1821, when Mexican independence was secured and the Texas Revolution in 1836, the Mexican Cession had formed approximately 42% 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 Grandemarker from the El Pasomarker area to Santa Femarker.


Total losses of 1848, Mexican view: white
The region includes all of the present-day states of Californiamarker, Nevadamarker, and Utahmarker; as well as portions of: The treaty also specified the international border between Texas and Mexico as being at the Rio Grandemarker (Río Bravo del Norte). Previously the portion of Texasmarker between the Nueces Rivermarker 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. Mexico had recognized neither.

The portion of New Mexicomarker east of the Rio Grande had been claimed by the Republic of Texas and is therefore usually shown as part of the Texas Annexation. In actuality New Mexico was never divided at the Rio Grande, was never controlled by the Republic of Texas, and was widely separated from Texas by the Comancheria which was not under either Anglo or Hispanic control. Stephen Kearny's U.S. Army of the West from Fort Leavenworth, Kansasmarker took New Mexico without resistance in 1846 early in the Mexican-American War. It was quickly provided with its own territorial government and was never governed by the State of Texas.

Subsequent organization and the North-South conflict

Soon after the war started and long before negotiation of the new US-Mexico border, the question of slavery in the territories to be acquired polarized the Northern and Southern United States in the bitterest sectional conflict up to this time, which lasted for a deadlock of four years during which the Second Party System broke up, Mormon pioneers settled Utahmarker, the California Gold Rush settled Californiamarker, and New Mexico under a federal military government turned back Texasmarker's attempt to assert control over territory Texas claimed as far west as the Rio Grandemarker. Eventually the Compromise of 1850 preserved the Union, but only for another decade. Proposals included:

  • The Compromise of 1850, proposed by Henry Clay in January 1850, guided to passage by Douglas over Northern Whig and Southern Democrat opposition, and enacted September 1850, admitted California as a free state including Southern California and organized Utah Territory and New Mexico Territory with slavery to be decided by popular sovereignty. Texas dropped its claim to the disputed northwestern areas in return for debt relief, and the areas were divided between the two new territories and unorganized territory. El Pasomarker where Texas had successfully established county government was left in Texas. No southern territory dominated by Southerners (like the later short-lived Confederate Territory of Arizona) was created. Also, the slave trade was abolished in Washington, DCmarker (but not slavery itself), and the Fugitive Slave Act was strengthened.


  1. Treaty of Guadalupe Hidalgo, Articles XII-XV
  2. Table 1.1 Acquisition of the Public Domain 1781-1867
  3.,M1 Quarterly of the Texas State Historical Association, January 1904

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