April 25, 1846 - April 25, 1846
In late March, 1846, the U.S. army began to take up positions on the northern side of the Rio Grande. In response, the Paredes government, having taken power at the beginning of the year, ordered General Mariano Arista, commander of the Army of the North, to attack the U.S. force. On April 25, 1846, Mexican troops attacked a squadron of U.S. dragoons commanded by Captain Seth Thornton near Fort Texas (later renamed Fort Brown, the site of modern-day Brownsville). The engagement left sixteen U.S. troops killed or wounded. Thornton, who was knocked unconscious when he fell off his horse, was at first listed among those killed in the incident. The remaining fifty-two soldiers were held at Matamoros and later exchanged for Mexican prisoners.
News of the incident reached Washington D.C. two weeks later. On May 11 President Polk asked Congress for a declaration of war, maintaining that a state of war between the two nations already existed. "Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil." Two days later, by overwhelming margins and with little debate, both houses voted to declare war.