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 3400 Mexican troops (a portion of the Army of The North) led by General Mariano Arista were engaged by a force of 2400 U.S. troops – the so called US Army of Observation near Palo Alto, Texas.
The battle occurred as a result of Mexican efforts to besiege a U.S. army installation, "Fort Texas", which the Mexicans viewed as having been built within the boundaries of Mexican Texas.
Before the opening shots of the engagement, many foreign observers were convinced that Mexico's larger and more professional army would easily defeat that of the United States. The opposite result, however, proved to be the case at Palo Alto. Mexican troops were positioned poorly, despite the orders of General Arista, and the so called "Flying Artillery" of the U.S. army wreaked havoc in the Mexican lines.
Toward the end of the day, some progress was made by the Mexicans in advancing against the U.S. line, but as became quite common as the war dragged on, the Mexicans instead chose to withdraw from the field, somewhat surprising Zachary Taylor.
The next engagement of the war, the Battle of Resaca de La Palma, was waged the next day.
The venue is now the Palo Alto Battlefield National Historic Site and is maintained by the U.S. National Park Service.
Bauer, K. Jack, "The Mexican-American War, 1846-1848"
- Palo Alto Battlefield National Historic Site (http://www.nps.gov/paal/index.htm)