Thornton Affair

The Thornton Affair, also known as the Thornton Skirmish, Thornton's Defeat, or Rancho Carricitos  was a battle in 1846 between the military forces of the United States and Mexico in disputed territory. The much larger Mexican force completely defeated the Americans. It served as the primary justification for U.S. President James K. Polk's call to Congress to declare war against Mexico, starting the Mexican-American War. 2000 Mexican soldiers attacked an American army patrol of 70 men, killing 16 Americans, wounding 5 and capturing 49.

Background
The event occurred around dusk on April 25, 1846, and continued into the early hours of April 26. Before and throughout the Mexican-American war, the state of Texas (which had recently joined the U.S.; previously it was the Republic of Texas) was considered by Mexico to be a rebelling Mexican province and not lawfully part of the U.S.. After Texas defeated Mexico in 1836, Mexico signed but refused to ratify the Treaties of Velasco, and warned that annexation of Texas by the U.S. meant war.

The United States regarded Texas as an independent nation (as did the European powers) but Mexico rejected negotiations on the matter. Both the US and Mexico claimed the uninhabited area between the Nueces River and the Rio Grande, where the Thornton Affair took place. By October 1845, 4000 U.S. troops, nearly half under orders of President Polk, were positioned on the north side of the Rio Grande. The Mexican garrison of Matamoros under Gen. F. Mejia consisted of the Zapadores (Sappers) Battalion, the 2d Light, 1st & 10th Line Infantry Regiments, the 7th Cavalry Regiment, Villas of the North Aux Cavalry, several Companies of Presidales and the Matamoros National Guards Battalion. One company of artillery served 20 guns. Two-three days after the arrival of the Americans, Mexican reinforcements (6th Line Infantry Regiment, Tampico National Guards Battalion & Tampico Marine Company) arrived, resulting in a total of 3,000 men. On the 11 & 14 April, Gen. br. P. Ampudia arrived with more reinforcements (2,200 men: Mexico Light and 8th Line Cavalry Regiments, 4th Line Infantry, Mexico, Puebla & Morelia Activo Battalions and 6 guns).

Battle
A small body of about 70 U.S. Dragoons commanded by Captain Seth Thornton was ordered to scout an area twenty miles (30 km) northwest of what later became Brownsville, Texas. Their mission was to determine whether or not the Mexican Army had crossed the Rio Grande for a possible attack on Fort Texas. On April 25, the Dragoons, acting on the advice of a local guide, investigated an abandoned hacienda. Two thousand Mexican soldiers under the command of Colonel Anastasio Torrejón were encamped in and around the hacienda, and fighting broke out. Vastly outnumbered, the U.S. unit was forced to surrender after several hours of resisting. One U.S. cavalryman was able to flee back to camp and reported to his superiors of the Mexican opening of hostilities.

Aftermath
In the fierce encounter, 16 U.S. Dragoons were killed and 5 wounded (including Captain Thornton). Mexican casualties are unknown. Thornton and 49 of his men were taken prisoner and held at Matamoros, Tamaulipas. Upon learning of the incident, President Polk asked for a Declaration of War before a joint session of the United States Congress, and summed up his justification for war by famously stating:


 * "The cup of forbearance had been exhausted even before the recent information from the frontier of the Del Norte [Rio Grande]. But now, after reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil. She has proclaimed that hostilities have commenced, and that the two nations are now at war.".

On May 13, 1846, Congress declared war on Mexico, despite the Mexican government's position that Thornton had crossed the border into Mexican Texas, which Mexico maintained began south of the Nueces River (the historical border of the province of Texas). Opposition also existed in the United States, with one senator declaring that the affair had been "as much an act of aggression on our part as is a man's pointing a pistol at another's breast". The ensuing Mexican-American War was waged from 1846-1848 with the loss of many thousands of lives and the loss to Mexico of all of its northern provinces. The Treaty of Guadalupe Hidalgo which ended the war established the Rio Grande as the border between Texas and Mexico, and led to Mexico recognising Texas as a part of the United States.