Treaty Of Guadalupe

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.