The Treaty of Guadalupe Hidalgo was not easily negotiated, for the disputes which underlay it went back to the question of Texas. Following the successful revolt of the Texans, including Mexicans who lived north of the Rio Grande, against the dictator Santa Ana, the Mexican government did not reconcile itself to the loss of this vast territory. Instead, it plotted and planned to recover Texas, by military force if necessary. The accession of Texas to the Union in 2 March 1845 poisoned relations between the United States and Mexico and set the stage for the Mexican-American War.
The American President, James K. Polk, wanted to resolve these and other issues peacefully, but he also wanted to acquire California for the Union. When the Mexican government rejected his emissary, John Slidell, the stage was set for war. The causus belli was the corssing of the Rio Grande by a body of Mexican troops. A skirmish broke out and several American soldiers were killed. America declared war and drove the Mexican force out of U.S. territory. A force under General Stephen Kearny took Arizona, New Mexico, and California, while General Zachary Taylor drove south into Mexico. While his campaign was a disappointment, General Winfield Scott was much more successful.