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 : 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 .