The origins of the Mexican-American War can largely be traced back to Texas winning its independence from Mexico in 1836. Following his defeat at the Battle of San Jacinto (4/21/1836), Mexican General Antonio López de Santa Anna was captured and forced to recognize the sovereignty of the Republic of Texas in exchange for his freedom. The Mexican government however, refused to honor Santa Anna’s agreement, stating that he was not authorized to make such a deal and that it still considered Texas a province in rebellion. Any thoughts the Mexican government had of recovering the territory quickly were eliminated when the new Republic of Texas received diplomatic recognition from the United States, Great Britain, and France.
During the next nine years, many Texans openly favored annexation by the United States, however Washington rejected the issue. Many in the North were concerned about adding another “slave” state to the Union, while others were concerned about provoking a conflict with Mexico. In 1844, Democrat James K. Polk was elected to the presidency on a pro-annexation platform. Acting quickly, his predecessor, John Tyler, initiated statehood proceedings in Congress before Polk took office. Texas officially joined the Union on December 29, 1845. In response to this action, Mexico threatened war, but was persuaded against it by the British and French.
After rebuffing an American offer to purchase the California and New Mexico Territories, tensions between the US and Mexico rose further in 1846, over a border dispute. Since its independence, Texas claimed the Rio Grande as its southern border, while Mexico claimed the Nueces River farther to the north. As the situation worsened, both sides sent troops to the area.
On the evening of April 25, 1846, while leading 70 US Dragoons to investigate a hacienda in the disputed territory between the rivers, Captain Seth Thornton stumbled upon a force of 2,000 Mexican soldiers. A fierce firefight ensued and 16 of Thornton’s men were killed before the remainder was forced to surrender. On May 11, 1846, Polk, citing the Thornton Affair asked Congress to declare war on Mexico. After two days of debate Congress voted for war—not knowing that the conflict had already escalated.