In the midst of the conflict in the town, while the Mexicans were disputing its possession from the windows of the strongly constructed houses, the ammunition of the brigade to which Grant was attached was exhausted, and it became necessary to send for a fresh supply. It was a service of extreme peril, and a volunteer was called for to perform it. Grant was a bold rider, and he promptly offered himself to execute the dangerous mission. Mounting a very spirited horse, he resorted to the Indian fashion of hanging at the side of his steed so that the body of the animal protected him against the shots from the windows, and he passed safely through the street. With a sufficient escort he succeeded in conveying a load of ammunition to the point where it was needed. Soon after the battle of Monterey, Grant's regiment was sent to Vera Cruz to reinforce the larger army that was to march under General Scott to the "Halls of the Montezumas." Lieutenant Grant, as a careful, substantial, and energetic officer, was selected for the important position of quartermaster of the Fourth Regiment. The army proceeded on its uninterrupted career of victory till the capital of Mexico was in its possession. The heights of Cerro Gordo were stormed and carried, and Grant, as usual, was in the thickest of the fight. The first considerable obstacle after the capture of Vera Cruz having been removed, the army proceeded on its march to the City of Mexico, occupying Jalapa and Castle Perote on the way; but at Puebla the forces were so reduced by sickness, death, and the expiration of enlistments as to compel a halt. For three months General Scott was compelled to wait for reinforcements; but when he could muster 11,000 effective men, a very small number for the conquest of a country, he resumed his march, and in August arrived in the vicinity of the capital. Outside of the causeways leading to the city were the strongholds of Chapultepec and Cherubusco, and batteries mounting a hundred guns. Chapultepec was a fortification one hundred and fifty feet above the average level of the ground. A front of nine hundred feet bristled with cannon. Behind it was a mill called El Molino del Rey, fortified and garrisoned, which defended the approach to the castle. The capture of this work was assigned to General Worth, to whose command the Fourth Regiment belonged. The assault was a desperate one, for it was "the last ditch" of the Mexicans; but it was carried, though the assailing force lost one-fourth of its number in the assault. "Second Lieutenant Grant behaved with distinguished gallantry," is the official report of his conduct. Though custom and the precedents of the service permitted the quartermaster to remain at a safe distance from actual fighting in charge of the baggage trains, Grant never availed himself of this immunity from personal peril, but retained his place with the regiment.