|Part of the Mexican–American War|
Rancho de Carricitos
|Commanders and leaders|
William J. Hardee
|80 ||1,600 |
|Casualties and losses|
1 fatally wounded
59 captured 
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 20 miles (32 km) west upriver from Zachary Taylor's camp along the Rio Grande. :48 The much larger Mexican force defeated the Americans in the opening of hostilities, and was the primary justification for U.S. President James K. Polk's call to Congress to declare war. :48
Although the United States had annexed Texas, both the US and Mexico claimed the area between the Nueces River and the Rio Grande. :11 Polk had ordered Taylor's Army of Occupation to the Rio Grande early in 1846 soon after Mexican President Mariano Paredes declared in his inaugural address to uphold the integrity of Mexican territory to the Sabine River. :26
Mariano Arista assumed command of the Division of the North on April 4 and arrived at Matamoros on April 24, making the total force there about 5000 men, and notified Taylor hostilities had commenced. :47 Arista promptly ordered General Anastasio Torrejón to cross the Rio Grande fourteen miles upstream at La Palangana. :48
Taylor received two reports on April 24 of Mexicans crossing the Rio Grande, the first crossing below his camp, the other a crossing upriver. :48 Taylor ordered Captain Croghan Ker to investigate downriver and Captain Seth B. Thornton with two Dragoon companies to investigate upriver. :48 Ker found nothing but Thornton rode into an ambush and his 80-man force was quickly overwhelmed by Torrejon's 1600, resulting in the capture of those not immediately killed. :48 Thornton's guide brought news of the hostilities to Taylor and was followed by a cart from Torrejón containing the six wounded, Torrejon stating he could not care for them. :48
In the fierce encounter, fourteen of Thornton's men were killed, six wounded and one was fatally wounded, while the rest were taken prisoner (including Captain Thornton and his second in command Captain William J. Hardee). :48 Mexican casualties are unknown. Torrejón continued on to the Matamoros-Point Isabel road, surprising Samuel H. Walker's Texas Rangers on April 28, before continuing on to Longoreno to cover the crossing of the main Mexican army. :48
Following the Battle of Palo Alto and the Battle of Resaca de la Palma, Arista and Taylor agreed to a prisoner exchange which resulted in the release of Thornton, Hardee and their men. :81 Thornton was killed on August 20, 1847 in an engagement at Churubusco outside Mexico City. With eerie symmetry, this soldier who was wounded at the war's opening act was killed in this last conflict of the war. :291  :139 
Upon learning of the incident, President James K. 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, the U.S. 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".  Congressman Abraham Lincoln demanded to know the "particular spot of soil on which the blood of our citizens was so shed" (the spot resolutions).  The ensuing Mexican–American War was waged from 1846 to 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 ended the war on February 2, 1848, and established the Rio Grande as the border between the U.S. and Mexico, and led to Mexico recognizing Texas as a part of the United States.
- Bauer, K.J., 1974, The Mexican War, 1846–1848, New York: Macmillan, ISBN 0803261071
- Tucker, Spencer (2013). The Encyclopedia of the Mexican-American War: A Political, Social, and Military History. ABC-CLIO. ISBN 9781851098538.
- Beschloss, Michael (2018). Presidents of War. Broadway Books. ISBN 9780307409614.
- "A Controversial War". Digital History. Archived from the original on May 7, 2008. Retrieved August 10, 2008.
- "Abraham Lincoln Protests the Mexican War". Digital History. Archived from the original on November 11, 2017. Retrieved November 11, 2017.