Historic drawing of Fort Brown
The site unofficially referred to as Fort Texas was a fieldwork, taking the rough shape of a six-sided star. Each packed-earth face of the fort extended from 125 to 150 yards. The walls were 9 feet in height, and 15 feet wide, with a moat, 20 feet wide and 8 feet deep circling the exterior. Inside, U.S. troops constructed a number of bomb-proofs and powder magazines to provide shelter from any incoming fire.
Mexican General Arista began positioning artillery and troops around the fort shortly after General Taylor departed on May 1, 1846. And, at 5 a.m. on May 3, 1846, Mexican forces opened fire on the fort from guns placed directly across the Rio Grande. Troops of the U.S. 7th Infantry quickly responded with their own artillery. When additional cannon fire erupted from Mexican positions up and down the river's bank, fort commander Jacob Brown pointed his guns into the city of Matamoros. Fire continued on both sides until well into the night.
In time, however, this artillery exchange gave way to a prolonged standoff. Despite the steady Mexican fire of May 3, the earthen walls of the fort withstood the impacts well. Mexican leaders apparently acknowledged the lack of success and, in the ensuing days, firing on the fort diminished considerably. Apparently believing that a charge on the fort would produce heavy casualties in his own ranks, Mexican General Pedro de Ampudia instead settled in for a more traditional siege in the hope that General Arista's army could prevent assistance from reaching the fort.
The cannonade from within the fort declined as well. Realizing that the shots directed on Matamoros were having minimal effect, Major Brown called for a halt to firing. Over the next several days, the U.S. troops conserved their limited ammunition, offered only brief flurries of return fire, and concentrated on shoring up the defenses of their post. Otherwise, the soldiers could do little but wait for General Taylor to march to the rescue.