On March 8, 1846, after a final attempt to pressure Mexico to settle on a boundary for Texas, Secretary of War William L. Marcy ordered Brigadier General Zachary Taylor to move his army, which included May's dragoon squadron, to the Rio Grande. Taylor's destination was the river's north bank, directly opposite the Mexican town of Matamoros, which stood at a natural choke-point and controlled access to well-traveled routes to the south. When Taylor refused to leave the region, Mexican cavalry ambushed a dragoon detachment under Captain Seth B. Thornton on April 25, 1846, which officially commenced hostilities. On May 8, 1846, the two main forces met at the Battle of Palo Alto, where May's squadron was held in reserve and mounted an unsuccessful cavalry charge.