<P> Eventually Nicholas Trist forced 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 . </P> <P> The U.S. Senate approved the treaty, rejecting amendments from both Jefferson Davis to also annex most of northeastern Mexico and Daniel Webster not to take even Alta California and Santa Fe de Nuevo Mexico . The United States also paid $15,000,000 ($482 million in 2016 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 debt to the U.S. at that time . </P> <P> The Mexican Cession as ordinarily understood (i.e. excluding lands claimed by Texas) amounted to 525,000 square miles (1,400,000 km), 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 km). If all of Texas had been seized, 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 km). </P> <P> Considering the seizures, including all of Texas, Mexico lost 54% of its pre-1836 territory in the Treaty of Guadalupe Hidalgo . For only fifteen years from 1821 (when Mexican independence was secured) and the Texan Revolt in 1836, the Mexican Cession (excluding Texas) 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 Roman Catholic 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 . </P>

The mexican cession included all of these states except