The climax to the regiment's participation in the Mexican War came on 13 September 1847 when the brigade the regiment belonged to was ordered to support the assault on the fortress of Chapultepec, the site of the Mexican National Military Academy. A pair of hand-picked, 250-man storming parties were formed, including a large number of Mounted Riflemen under CPT Benjamin S. Roberts. During the charge, a party of US Marines began to falter after their officers were lost, so LT Robert M. Morris of the regiment quickly took charge and led them to the top. While the fortress was being stormed, other elements of the regiment captured a Mexican artillery battery at the bottom of the castle. Leading the American forces, the regiment stormed into Mexico City at 1:20 pm. At 7:00 am on 14 September 1847, Sergeant James Manly of F Company and Captain Benjamin Roberts of C Company raised the National Colors over the National Palace while Captain Porter, commander of F Company, unfurled the regimental standard from the balcony.