"From your known high character, both as a public officer and private citizen, I was strengthened in my hope that some arrangement could be made by which friendly relations might be maintained on the frontier, until a final settlement of the question of boundary...But if such is not to be the case - if hostilities are to ensue - the responsibility must rest with them who actually commence them."

  True to his word, General Mariano Arista directed Mexican troops to cross the Rio Grande. On April 25, 1846, at Rancho de Carricitos (about 25 miles northwest of present-day Brownsville), 2,000 men under command of General Torrejon ambushed a squad of U.S. dragoons led by Captain Seth Thornton. General Taylor immediately sent a report of the "Thornton Affair" to President Polk. After consulting with his cabinet on Sunday, Polk declared war. In the days immediately following the ambush of Captain Thornton and his men, General Taylor became increasingly troubled by reports that the Mexican army, split into two divisions, had crossed both above and below the fort, with a view to attacking Taylor's vulnerable supply depot at Point Isabel, located on the Texas coast about twenty miles northeast of Fort Texas.

  On that very same day the Mexican forces in Matamoros laid siege to the tiny garrison left behind at Fort Texas. Although greatly outnumbered and unsure when the rest of the "Army of Occupation" would be able to come to the rescue, the garrison, under command of Captain Edgar Hawkins, who took over after Brown was injured, refused to surrender. After loading a large number of supply wagons, the army set out from Point Isabel to relieve Fort Texas (also called Fort Taylor). The next afternoon, General Taylor found the road to the fort blocked by the Mexican army, under command of General Arista, at a place called Palo Alto, about eight miles north of the Rio Grande. There, the first major battle of the Mexican War took place. The engagement resulted in heavy losses for the Mexicans and the need to pull back to a more easily defensible position. Since they were not driven from the field, the Americans claimed a victory, although it was not a definitive one.