Battles on the Rio Grande
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With the outbreak of hostilities at Rancho de Carricitos, both the U.S. and Mexican armies accepted war had commenced. Neither waited for a formal declaration. General Mariano Arista began shuttling troops across the Rio Grande to besiege the isolated U.S. post, Fort Texas.
General Zachary Taylor expected just such a move. On May 1, 1846, Taylor marched most of his troops to Point Isabel, a portage area on the Gulf of Mexico. It was there the veteran U.S. general would meet an incoming naval fleet carrying supplies needed to withstand a prolonged siege. General Taylor left Major Jacob Brown, the U.S. Seventh Infantry, and portions of the Third Artillery—some 550 men—to hold the post on the river.
General Arista was not able to prevent Taylor’s departure for the coast, but moved quickly to surround the U.S. outpost. On May 3, 1846, Mexican artillery opened fire on Fort Texas from all directions, hoping to force a quick surrender of the defenders.
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The first clash
General Taylor soon marched to assist his besieged men. After pausing to improve defenses at his supply depot, known as Fort Polk, he set out on May 7th with 2,300 troops and 200 supply wagons to break the siege of Fort Texas.
General Arista moved to block this advance, positioning 3,200 troops across the Matamoros road as it crossed the broad prairie of Palo Alto. There, on the afternoon of May 8, 1846, he engaged the U.S. force in a fierce artillery battle—the first major clash of the war.
Mexican forces stood their ground but suffered heavy casualties. On the morning of May 9, General Arista withdrew several miles to the brush covered banks of Resaca de la Palma. Taylor's troops pursued the Mexican forces and engaged them that afternoon in the battle of Resaca de la Palma. There, U.S. soldiers overran Arista's lines, forced Mexican troops to retreat across the Rio Grande, and ended the six-day siege of Fort Texas.
Taylor enters Matamoros
On May 18, 1846, Taylor’s troops forded the Rio Grande and entered the city of Matamoros. The occupation took place without a fight. Mexican forces had determined the city could not be defended and had withdrawn down the road leading to Monterrey. The lower Rio Grande Valley had been conceded to the U.S. Army.