In the Spring of 1846, disputes over the ownership and boundaries of Texas thrust the United States and Mexico into war.

On December 29, 1845, U.S. President James K. Polk fulfilled a long-standing campaign promise by welcoming the former Republic of Texas as the 28th state in the Union. But Mexicans insisted that Texas was rightfully part of their country. Although settlers in this territory had claimed independence from Mexico since 1836, Mexican leaders had never recognized Texan sovereignty and still harbored faint hopes of controlling the wayward province. They denounced the U.S. annexation as an act of aggression against Mexico.

Tensions heightened when U.S. President Polk announced that the Rio Grande formed the boundary of Texas. While Texans championed this river as their boundary, Mexico mapped Texas as a much smaller region--bounded in part by the Nueces River. Left in dispute was a huge expanse of land between the two rivers.

In July of 1845, Polk sent an army led by General Zachary Taylor to Corpus Christi, on the banks of the Nueces River. The troops were officially dispatched to help defend Texas from a potential Mexican attack on Texas. But they also represented a display of power as a U.S. negotiator headed to Mexico.

The advance of U.S. troops and Polk’s new demand that Mexico sell its New Mexico and California territories infuriated Mexican President Joaquin Herrera, who refused to meet with the American envoy. But Herrera’s willingness just to allow the envoy into Mexico brought about his downfall. In December of 1845, General Mariano Paredes y Arrillaga overthrew Herrera, installed himself as President, and vowed to discuss nothing but the return of Texas.

Polk continued to exert pressure. In January 1846, he ordered General Taylor to claim the Rio Grande as the U.S. boundary.In March, Taylor led 4,000 troops to the river’s edge, across from the Mexican city of Matamoros, and began construction of Fort Texas, the earthen fieldwork that would serve as a U.S. base.

General Paredes was not swayed by this show of force. He expelled the U.S. envoy from Mexico, declared his willingness to fight, and sent thousands of troops to the city of Matamoros. In April 1846, he appointed General Mariano Arista as the commander of this growing force.

The U.S. President hesitated to respond with hostilities against Mexico. Many political foes within the United States questioned his claims to the Rio Grande boundary, and Polk realized that a move to cross the Rio Grande or fire the first shots of war might sway public opinion against his cause. Instead, he waited for Mexican forces to initiate combat north of the river so that he could portray the conflict as one of national defense.

Mexican leaders viewed the U.S. occupation of the Rio Grande as an invasion deep into their territory and did not hesitate to cross the river. On April 25, 1846, Mexican troops overwhelmed a U.S. scout party on the north bank of the river, at Rancho de Carricitos. When word of this clash reached Washington D.C. on May 10, Polk announced that Mexico had "spilled American blood upon the American territory" and demanded a response. On May 13, 1846, as a spirit of nationalism swept away recent debates, Congress complied and declared war.

A Mexican War Overview
A Fight for Texas | Battles Along the Rio Grande | An Extended War | Peace and After