In November 1835, the northern part of the Mexican state of Coahuila-Tejas declared itself in revolt against Mexico's new centralist government headed by President Antonio López de Santa Anna. By February 1836, Texans declared their territory to be independent and that its border extended to the Rio Grande rather than the Rio Nueces that Mexicans recognized as the dividing line. Although the Texans proclaimed themselves citizens of the Independent Republic of Texas on April 21, 1836 following their victory over the Mexicans at the Battle of San Jacinto, Mexicans continued to consider Tejas a rebellious province that they would reconquer someday.
In December 1845, the U.S. Congress voted to annex the Texas Republic and soon sent troops led by General Zachary Taylor to the Rio Grande (regarded by Mexicans as their territory) to protect its border with Mexico. The inevitable clashes between Mexican troops and U.S. forces provided the rationale for a Congressional declaration of war on May 13, 1846.
Hostilities continued for the next two years as General Taylor led his troops through to Monterrey, and General Stephen Kearny and his men went to New Mexico, Chihuahua, and California. But it was General Winfield Scott and his army that delivered the decisive blows as they marched from Veracruz to Puebla and finally captured Mexico City itself in August 1847.
Mexican officials and Nicholas Trist, President Polk's representative, began discussions for a peace treaty that August. On February 2, 1848 the Treaty was signed in Guadalupe Hidalgo, a city north of the capital where the Mexican government had fled as U.S. troops advanced. Its provisions called for Mexico to cede 55% of its territory (present-day Arizona, California, New Mexico, Texas, and parts of Colorado, Nevada and Utah) in exchange for fifteen million dollars in compensation for war-related damage to Mexican property.
Other provisions stipulated the Texas border at the Rio Grande (Article V), protection for the property and civil rights of Mexican nationals living within the new border (Articles VIII and IX), U.S. promise to police its side of the border (Article XI), and compulsory arbitration of future disputes between the two countries (Article XXI). When the U.S. Senate ratified the treaty in March, it deleted Article X guaranteeing the protection of Mexican land grants. Following the Senate's ratification of the treaty, U.S. troops left Mexico City.