Mexican Cession - War and Treaty

War and Treaty

The cession of this territory from Mexico was a major goal of the war. Alta California and Santa Fe de Nuevo 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 City, 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, explicitly redefining the border between Mexico and the United States, in early 1848 after President Polk had already attempted to recall him from Mexico as a failure. Although Mexico did not overtly cede any land under the treaty, the redefined border had the effect of transferring Alta California and Santa Fe de Nuevo Mexico to the control of the United States. Equally important, the new border also acknowledged Mexico's loss of Texas, both the core eastern portion and the western claims, neither of which had been formally recognized by Mexico until that time.

The U.S. Senate approved the treaty, rejecting amendments from Jefferson Davis to also annex most of northeastern Mexico and by Daniel Webster not to take even Alta California and Santa Fe de Nuevo Mexico. The United States also paid $15,000,000 ($298,310,309 in 2005 dollars) for the land, and agreed to assume $3.25 million in debts to US citizens. While technically the territory was purchased by the United States, the $15 million payment was simply credited against Mexico's enormous debt towards the U.S. at that time.

The Mexican Cession as ordinarily understood (i.e. excluding lands claimed by Texas) amounted to 525,000 square miles (1,400,000 km2), or 14.9% of the total area of the current United States. If the disputed western Texas claims are also included, that amounts to a total of 750,000 square miles (1,900,000 km2). If all of Texas is included (since Mexico had not previously acknowledged the loss of any part of Texas), the total area ceded under this treaty comes to 915,000 square miles (2,400,000 km2).

Considering everything including all of Texas, Mexico lost 55% of its pre-1836 territory in the Treaty of Guadalupe Hidalgo. For the fifteen years between 1821 (when Mexican independence was secured) and the Texas Revolution in 1836, the Mexican Cession (excluding Texas) 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 Grande from the El Paso area to Santa Fe.

As of 2012, the states of California, Nevada, Utah, Arizona and New Mexico had a total population of 51,797,185 out of 313,000,000 nationwide, or 16.5% of the US population.