The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, ended the Mexican War, recognized the annexation of Texas to the United States (consummated nearly three years before), and ceded to the United States Upper California (the modern state of California) and nearly all of the present American Southwest between California and Texas. The treaty traced the boundary between the United States and Mexico from the Gulf of Mexico up the main channel of the Rio Grande to the southern boundary of the Mexican province of New Mexico. The line followed the southern boundary of New Mexico to its western boundary and north to the first branch of the Gila River, then down the Gila to its intersection with the Colorado River, and finally along the old Spanish-Mexican division line between Upper and Lower California. The exact boundary was to be surveyed and marked by a joint commission to be appointed by the two governments within a year.
The negotiation of the treaty presented many knotty problems, mostly caused by the Mexicans' sense of honor and their stubborn refusal to admit defeat. After Gen. Winfield Scott had captured Veracruz and pushed into the interior of Mexico, President James K. Polk and Secretary of State James Buchanan sent Nicholas P. Trist, from the Department of State, to join him in the hope that with plenipotentiary powers and full instructions the two could take advantage of any crack in Mexican resolve to push through negotiations. Scott and Trist managed to establish communications with the Mexican government by way of British legation officials in Mexico City, who were anxious to see peace return before Mexico was entirely crushed. For a time the Mexican president, Antonio López de Santa Anna, seemed willing to consider a negotiated peace, and Scott decided to "encourage" him by occupying the Valley of Mexico and threatening Mexico City, goals he accomplished after several bloody but victorious battles. In August-September 1847 Trist met with Mexican commissioners to discuss terms, but the talks broke down, and Scott was forced to occupy Mexico City. Thereupon Santa Anna resigned, and the rest of the Mexican government fled to a provisional capital about a hundred miles to the north. At this point an indefinite deadlock might have resulted, for the weak Mexican administration succeeding Santa Anna dared not act, and the Mexican congress, which split into a number of peace and war factions, did everything possible to avoid responsibility. By the end of November, after weeks of jockeying, Trist, aided by British diplomats, managed to persuade the Mexicans to send a peace commission to Mexico City and at least exchange proposals. At that point a dispatch arrived from the impatient Polk instructing Trist to give up the effort and return home. Pressed by Scott and the other American generals and by the British, Trist decided to disobey his orders and stay.
The active negotiations, which lasted about a month, were delayed at every point by haggling over details and slow correspondence between the Mexican commissioners and their government, a hundred miles away, which resisted every concession. At first the Mexicans would recognize Texas territory only to the Nueces River. They also resisted the cession of New Mexico, and a particularly bitter argument arose over the cession of San Diego, which they denied had ever been a part of Upper California. The American purchase price of $15 million represented half of the original Mexican demand. When Polk received the completed treaty he was affronted by Trist's disobedience and stopped his salary. On further consideration, he decided to submit the treaty to the Senate because it met his minimum instructions concerning the boundary and because any continuation of the war risked serious disunion in the country during an election year. The nation received the treaty with relief, and the Senate approved it after a few minor changes, with a few votes to spare over the required two-thirds majority. The execution of the treaty was generally satisfactory to both sides, except for its requirement that the United States prevent Indian raids into Mexico, an almost impossible task on a long, unsettled frontier. As Americans came to desire more Mexican territory south of the Gila River for a railroad route, a new treaty, the Gadsden Purchase, was negotiated in 1853 to make the desired changes. See also BOUNDARIES.