On the night of 13–14 September 1847, Rea's forces entered the city with 4,000 men.[1]: 329 The U.S. forces held the convent, Fort Loretto, and the citadel of San José.[1]: 329 Lieutenant-Colonel Samuel W. Black, commander of the First Pennsylvania, was put in command of the citadel, which also served as a hospital for 1,800 sick and wounded soldiers. The Mexicans drove off most of the city's cattle, but Childs was able to save enough to keep from starvation.[1]: 329 Rea demanded the garrison's surrender on 16 September, but Childs refused, leading Rea to attack San José, unsuccessfully.[1]: 329 Childs repulsed a second attack on 18 Sept.[1]: 329
Santa Anna arrived on 22 September, and launched a 500-man attack on the convent, once again unsuccessfully, yet called for Childs to surrender, which he refused.[1]: 329 The attacks continued from 27 Sept. until 1 Oct.[1]: 329
At the end of September, Santa Anna departed with most of the Mexican forces to confront General Joseph Lane's relief column.[1]: 329 Santa Anna was defeated at the Battle of Huamantla, allowing Lane to raise the siege on 12 Oct.[1]: 331