The Thornton Affair occurred in 1846 as tensions escalated between the United States and Mexico over territorial claims. Although the United States had annexed Texas, both nations disputed control of the area between the Nueces River and the Rio Grande. President James K. Polk had ordered Zachary Taylor's "Army of Occupation" to the Rio Grande early in 1846, following Mexican President Mariano Paredes's inaugural declaration that he would uphold Mexican territorial integrity to the Sabine River. This positioning of American forces in the contested region set the stage for direct military confrontation.
The battle took place in 1846 approximately 20 miles (32 km) west upriver from Taylor's camp along the Rio Grande. Mexican General Mariano Arista assumed command of the Division of the North on April 4 and arrived at Matamoros on April 24, bringing the total Mexican force there to about 5,000 men. Arista notified Taylor of his arrival and subsequently ordered his subordinate, General Anastasio Torrejón, to cross the Rio Grande fourteen miles upstream at La Palangana. The Mexican force, described as much larger than the American contingent, engaged Taylor's troops in what became known alternatively as the Thornton Skirmish, Thornton's Defeat, or the Battle of Rancho Carricitos.
The Mexican forces achieved victory in this opening engagement of hostilities. The American defeat at the Thornton Affair proved historically significant as it served as the primary justification for President Polk's call to Congress to declare war on Mexico. This battle thus marked an important escalation in the conflict and provided the impetus for formal U.S. entry into what would become the Mexican-American War.
The Mexican-American War (1846–1848) grew from the annexation of Texas (1845) and a disputed border between Texas and Mexico at the Rio Grande. President James K. Polk ordered US troops under General Zachary Taylor into the contested zone; after a skirmish that killed American soldiers, Congress declared war in May 1846. US forces won a series of engagements — Palo Alto, Monterrey, Buena Vista — before General Winfield Scott led an amphibious landing at Veracruz and an overland campaign to Mexico City, which fell in September 1847. The Treaty of Guadalupe Hidalgo (February 1848) transferred California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming to the United States in exchange for $15 million and assumption of $3.25 million in claims — roughly 525,000 square miles, a 67 percent expansion of US territory. The war's outcome immediately reopened the slavery question: the Wilmot Proviso, debated throughout the war, proposed banning slavery from any territory acquired from Mexico, foreshadowing the sectional crisis of the 1850s.
11 US killed, 5 wounded, 47 captured; Mexican casualties unknown
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