With the defeat of its army and the fall of its capital, Mexico entered into negotiations to end the war. The treaty called for the U.S. to pay US$15 million to Mexico and to pay off the claims of American citizens against Mexico up to US$5 million. It gave the United States the Rio Grande as a boundary for Texas, and gave the U.S. ownership of California and a large area comprising roughly half of New Mexico, most of Arizona, Nevada, and Utah, and parts of Wyoming and Colorado. Mexicans in those annexed areas had the choice of relocating to within Mexico's new boundaries or receiving American citizenship with full civil rights.
The U.S. Senate advised and consented to ratification of the treaty by a vote of 38–14. The opponents of this treaty were led by the Whigs, who had opposed the war and rejected Manifest destiny in general, and rejected this expansion in particular. The amount of land gained by the United States from Mexico was increased as a result of the Gadsden Purchase of 1853, which ceded parts of present-day southern Arizona and New Mexico to the United States.
The peace talks were negotiated by Nicholas Trist, chief clerk of the US State Department, who had accompanied General Winfield Scott as a diplomat and President Polk's representative. Trist and General Scott, after two previous unsuccessful attempts to negotiate a treaty with General José Joaquín de Herrera, determined that the only way to deal with Mexico was as a conquered enemy. Nicholas Trist negotiated with a special commission representing the collapsed government led by Don
José Bernardo Couto, Don Miguel de Atristain, and Don Luis Gonzaga Cuevas of Mexico.