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 (8 km) from the modern-day city of Brownsville, Texas. A force of some 3,400 Mexican troops – a portion of the Army of The North – led by General Mariano Arista engaged a force of 2,400 United States troops – the "Army of Observation."