The Thornton-Affair refers to a pivotal event that took place in the early 19th century, specifically on April 25, 1846, which significantly contributed to the escalation of tensions leading to the Mexican-American War. This incident involved a skirmish between the United States Army, under the command of Captain Seth B. Thornton, and Mexican forces near the Rio Grande, which was then the disputed border between the United States and Mexico.
Following the annexation of Texas by the United States in 1845, the border dispute between the U.S. and Mexico intensified. The U.S. claimed the border was the Rio Grande, while Mexico maintained that the border was the Nueces River, further north. This area between the two rivers, known as the Nueces Strip, was a no man's land where conflicts often occurred.
Captain Seth B. Thornton was dispatched with a force of 63 dragoons to investigate reports of Mexican troops crossing the Rio Grande. On the evening of April 25, Thornton and his men were ambushed by a much larger Mexican cavalry unit. The American forces were quickly overwhelmed, resulting in the capture of Thornton and many of his men, while others were killed or wounded. This event was reported back to President James K. Polk, who used it as justification to ask Congress for a declaration of war against Mexico.
The skirmish was a catalyst for the Mexican-American War, which began officially on May 13, 1846, when the U.S. Congress declared war. The incident was highlighted by President Polk in his message to Congress, where he famously stated that "American blood has been shed on American soil," despite the fact that the location was disputed territory.
The Thornton-Affair is often cited as one of the primary reasons for the outbreak of the Mexican-American War. It demonstrated the volatility of the border region and the readiness of both nations to engage in military action over territorial disputes. The war ended with the Treaty of Guadalupe Hidalgo, which established the Rio Grande as the international boundary between the U.S. and Mexico.