In early 1846, U.S. General Zachary Taylor led the "Army of Observation” to the Rio Grande River, recognized as the Texas border by the Americans. On April 25, Mexican General Anastasio Torrejón's troops exchanged fire with U.S. Dragoons (cavalry) north of the Rio Grande. General Mariano Arista congratulated Torrejón, saying it was "preliminary of glorious deeds that [Mexico's] happy sons will in future present to her."

Taylor took most of his troops and raced to the coast for provisions to fortify the remaining soldiers at the newly-built Fort Texas, commanded by Arkansan Major Jacob Brown. While returning to the fort on May 8, Taylor met Arista's army blocking the road at Palo Alto; the indecisive battle resulted in a Mexican withdrawal. The next morning, the Americans met Arista's forces in battle at Resaca de la Palma and won. During the siege of Fort Texas, Brown was wounded by a cannon ball and died. Shortly after, Taylor changed the name of Fort Texas to Fort Brown, site of today's Brownsville, Texas.

On May 11, President Polk carried battle news to the U.S. Congress, declaring that Mexico had "invaded our territory and shed American blood upon American soil." Congress declared war.