"The drift toward war with Mexico had begun in 1845 when the United States annexed the Republic of Texas as a new state. In 1836, the Mexicans had fought an unsuccessful war with Texans to keep them from breaking away to become an independent nation. Since then, Mexico had refused to recognize the independence of Texas or the Rio Grande River as an international boundary. In January 1946, fearing the Mexicans would respond to U.S. annexation by asserting control over disputed territory in southwestern Texas, President James Polk (1795-1849) ordered General Zachary Taylor to move a force into Texas to defend the Rio Grande border." - History.com
"At Palo Alto, Taylor tested the superiority of the so-called "flying artillery" developed by Maj. Samuel Ringgold, who was mortally wounded in the battle. Guns were mounted on light carriages drawn by specially trained teams of horses and could be moved quickly for tactical advantage. Although soldiers on both sides clamored for the traditional bayonet charge across the field, the artillery duel dominated the action. The last maneuver at Palo Alto, nonetheless, was a desperate Mexican charge at sunset. Afterwards, the action tapered off. To neutralize the American artillery advantage at Palo Alto, the Mexican army moved southward at dawn on May 9 to Resaca de la Palma. Before it could regroup, Taylor charged and defeated Arista's army. Shortly, the Americans captured Matamoros, and Arista retreated southward toward Monterrey. Other notables at Palo Alto included Gen. Pedro de Ampudia, Gen. Anastasio Torrejón, Col. José López Uraga, Gen. William J. Worth, Lt. Ulysses S. Grant, and Texas Ranger captain Samuel H. Walker." - Texas State Historical Association
The Battle of Palo Alto was the first battle fought in the war, where General Taylor marched his American troops to defend the Rio Grande border, which the Mexicans refused to recognize as an international border. Palo Alto was the first of many decisive American victories over the Mexican military.