The Battle of Palo Alto was the first major battle of the Mexican–American War and was fought on May 8, 1846, on disputed ground five miles from the modern-day city of Brownsville, Texas. A force of some 3,700 Mexican troops – most of the Army of The North – led by General Mariano Arista engaged a force of approximately 2,300 United States troops – the Army of Occupation led by General Zachary Taylor.
On April 30, following the Thornton Affair, Mexican General Mariano Arista's troops began to cross the Rio Grande. On May 3, the troops began to besiege the American outpost at Fort Texas. Taylor marched his Army of Occupation south to relieve the siege. Arista, upon learning of his approach, diverted many of his units away from the siege to meet Taylor's force. The battle took place on May 8, three days before the formal declaration of war on Mexico by the United States. Arista ordered two cavalry charges, first against the American right flank and later against the left. Both were unsuccessful. The American victory is widely attributed to superior artillery, as the U.S. "light" artillery was much more mobile and accurate than that of the Mexican forces.