<P> Anti-slavery elements opposed that position and fought for the exclusion of slavery from any territory absorbed by the U.S. In 1847, the House of Representatives passed the Wilmot Proviso, stipulating that none of the territory acquired should be open to slavery . The Senate avoided the issue, and a late attempt to add it to the Treaty of Guadalupe Hidalgo was defeated . </P> <P> The Treaty of Guadalupe Hidalgo was the result of Nicholas Trist's unauthorized negotiations . It was approved by the U.S. Senate on March 10, 1848, and ratified by the Mexican Congress on May 25 . Mexico's cession of Alta California and Nuevo México and its recognition of U.S. sovereignty over all of Texas north of the Rio Grande formalized the addition of 1.2 million square miles (3.1 million km) of territory to the United States . In return the U.S. agreed to pay $15 million and assumed the claims of its citizens against Mexico . A final territorial adjustment between Mexico and the U.S. was made by the Gadsden Purchase in 1853 . The sale of this territory was a contributing factor in the final fall of Santa Anna in Mexico for having sold Mexican patrimony . </P> <P> As late as 1880, the "Republican Campaign Textbook" by the Republican Congressional Committee described the war as "Feculent, reeking Corruption" and "one of the darkest scenes in our history--a war forced upon our and the Mexican people by the high - handed usurpations of Pres 't Polk in pursuit of territorial aggrandizement of the slave oligarchy ." </P> <P> The war was one of the most decisive events for the U.S. in the first half of the 19th century . While it marked a significant waypoint for the nation as a growing military power, it also served as a milestone especially within the U.S. narrative of Manifest Destiny . The resultant territorial gains set in motion many of the defining trends in American 19th - century history, particularly for the American West . The war did not resolve the issue of slavery in the U.S. but rather in many ways inflamed it, as potential westward expansion of the institution took an increasingly central and heated theme in national debates preceding the American Civil War . Furthermore, in doing much to extend the nation from coast to coast, the Mexican--American War was one step in the huge migrations to the West of Americans, which culminated in transcontinental railroads and the Indian wars later in the same century . </P>

The united states gained control of california by