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On the morning of April 25, Captain Seth Thornton’s force of U.S dragoons marched twenty miles upriver from Fort Texas to investigate reports that Mexican troops had crossed the Rio Grande River. The search for the enemy brought the Americans to Rancho de Carricitos, a farm field surrounded by impenetrable chaparral thickets. Suddenly, a substantial Mexican force surrounded the farm and blockaded the Americans only exit. After an unsuccessful charge, Thornton’s men dismounted and attempted to cut their way out of the chaparral. This too failed. Taking heavy casualties, the Americans surrendered. Brigadier General Anastasio Torrejon’s force headed east where it soon tangled with General Zachary Taylor’s forces in the Battles of Palo Alto and Resaca de la Palma. Thornton’s command was later part of a prisoner exchange. 

When reports of the Thornton Affair reached Washington DC, President James Polk declared, “Mexico has invaded our territory and shed American blood upon American soil.” Despite some opposition, Congress agreed and overwhelmingly declared war on Mexico. Mexico contended that Thornton’s troops had been the invaders since Mexico claimed the territory between the Rio Grande and Nueces rivers. Military success at Rancho de Carricitos also provided the Mexicans with a level of confidence in the ability of their ability to withstand U.S. military might. Today, the constant shifting of the Rio Grande River has made the exact site where the Mexican War “started” uncertain.