Short Term Causes of the Mexican American War

Every American schoolchild learns the motto, “Remember the Alamo!” Fewer remember that the landmark battle, part of Texas’ rebellion against Mexico, was only an early salvo in what became a decade-long struggle over the fate of Texas, eventually the U.S.’s largest contiguous state. The Mexican-American War, which finally decided the issue, lasted only two years but had enduring impact. The war’s short-term causes include U.S. annexation of Texas, the failure of John Slidell’s diplomatic mission, and the Matamoros ambush, which fueled President James K. Polk's expansionism.

Mexico never recognized the Republic of Texas, though the two governments seemed to be moving toward reconciliation by the early 1840s. Many American Texans had been aiming for annexation by the U.S. for years, and pro-annexation candidate Anson Jones decisively won the Texas 1844 presidential election. The Mexican government saw the loss of Texas as foreshadowing similar challenges in California and New Mexico, suggests historian Jesús Velasco-Márquez of Instituto Tecnológico Autónomo de México. When the U.S. formally annexed Texas in 1845, Mexican leadership reacted decisively, sending troops to the Rio Grande. Foreign relations minister Manuel de la Peña y Peña said his nation was “gravely offended.”

President Polk sent Slidell as envoy to Mexico City in 1845, although Velasco-Márquez argued that the Polk administration intended Slidell to fail, because they did not want a diplomatic solution. Slidell was charged with negotiating the sale of California, offending Mexican officials, who expected discussions to begin with Texas. Mexican President José Joaquín de Herrera, a moderate, had limited support for diplomacy with the U.S., though he attempted private discussions alongside his public rebukes; his successor, hardliner Mariano Paredes, took a stronger stance. His Congressional decree of 1846 made it clear that Mexico would brook no U.S. aggression.

Before Slidell had even returned home, President Polk had sent General Zachary Taylor with troops to the Texan side of the Rio Grande in a show of protecting the newly annexed territory’s border. Taylor’s position across the river from Matamoros put his troops squarely in the disputed territory. General Torrejon led 2,000 Mexican troops to ambush a U.S. squad led by Captain Seth Thornton; 14 American dragoons died, according to the Descendants of the Mexican War Veterans.

Texas annexation had been a major issue in the 1844 U.S. presidential race, and Polk’s victory rested, in part, on his assurances that the nation would push westward, adding Oregon, Texas and California to its territory. Indiana University’s David M. Pletcher argues that the Americans wanted this provocative move to inflame the situation, calling the raid from Matamoros the excuse that Polk needed to declare war. He could not afford to delay, because U.S. acquisition of California would add pressure to Great Britain, with whom he was negotiating over Oregon. Polk addressed a joint session of Congress the day after he received Taylor’s report of the ambush at Matamoros to request a declaration of war.