Santa Anna resigned as president, and two months passed before a new Mexican government was organized. Nicholas Trist, chief clerk of the U.S. Department of State, signed a peace treaty at Guadalupe Hidalgo on February 2, 1848. Mexico recognized the Rio Grande as the southern boundary of Texas, and ceded its territory north of an irregular line extending from El Paso to the Pacific Ocean. The United States agreed to pay $15,000,000 for the territory ceded, and to assume up to $3,250,000 of claims previously made by United States citizens against Mexico.

Polk accepted the treaty and forwarded it to the Senate. The Senate ratified it on March 10 by a vote of 38 to 14. Opposition came from two groups—those who feared the extension of slavery to the territory acquired, and those who wanted to annex all of Mexico or a larger part of it. The Mexican Congress ratified the treaty on May 25. United States casualties in the Mexican War numbered 1,721 who were killed or died of wounds; 11,155 who died of disease; 361 dead from other causes; and 4,102 wounded. Mexican casualties were greater, but the number is not known.