During the Mexican War, General Winfield Scott was accompanied by Nicholas Trist, a State Department official empowered to negotiate for the United States. Trist was initially rebuffed by the Mexican authorities, who objected particularly to the designation of the Rio Grande as the southern border of Texas. Hostilities resumed. After Mexico City fell and General Santa Ana was forced from the presidency, President Polk recalled Trist to Washington, but Trist instead remained, sending Polk a letter of "explanation," and with the support of General Winfield Scott, resumed with negotiations at Guadalupe Hidalgo, near Mexico City.
The resulting treaty, signed on February 2, 1848, and formally proclaimed on July 4, 1848, provided for the following:
The United States received all of the land originally sought by John Slidell, including present-day New Mexico, Arizona, California, Texas and parts of Colorado, Utah and Nevada; this area is often called the "Mexican Cession"
The Mexicans received $15 million for those lands and were relieved of responsibility for claims by American citizens (about $3 million)
The border between the two nations was fixed at the Rio Grande
The United States pledged to protect the rights of Mexicans living in the newly acquired areas
Both nations agreed to submit future disputes to arbitration.
President Polk expressed some unhappiness with the results of the negotiations, claiming that Trist had violated his instructions by not insisting upon taking more territory from Mexico. The pro-war forces in Congress were hoping to take over all of Mexico (the so-called "All Mexico movement").
The country was split on the issue of ratification. A reluctant Polk realized that the bulk of his aims had been accomplished and submitted the treaty to the Senate, where it was narrowly approved along sectional lines in March 1848. U.S. occupation forces were withdrawn from Mexico City in June.
The results of the war touched off a period of expansion in the United States and a wave of recrimination in Mexico.