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 ownership 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 in early 1846, following Mexican President Mariano Paredes's inaugural address declaration that he would uphold Mexican territorial integrity to the Sabine River. This positioning of American forces in contested territory set the stage for direct military confrontation.
The battle took place approximately 20 miles west upriver from Taylor's camp along the Rio Grande. Mexican General Mariano Arista had assumed command of the Division of the North on April 4 and arrived at Matamoros on April 24 with approximately 5,000 men. Arista notified Taylor of his presence and subsequently ordered his subordinate, General Anastasio Torrejón, to cross the Rio Grande fourteen miles upstream at La Palangana. The much larger Mexican force engaged American military units in what became known by several names: the Thornton Affair, the Thornton Skirmish, Thornton's Defeat, or the Battle at Rancho Carricitos.
The Mexican forces achieved a decisive victory in this opening engagement of hostilities between the two nations. This American defeat proved historically significant as it became the primary justification for President Polk's appeal to Congress for a declaration of war against Mexico, ultimately leading to 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
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