Scott’s army was rapidly concentrated about Ayotla and other points near the eastern end of Lake Chalco.  Reconnoissances were made up to within gun-shot of El Penon, while engineers were seeking a route by the south side of Lake Chalco to flank the city, and come upon it from the south and south-west.  A way was found around the lake, and by the 18th of August troops were in St. Augustin Tlalpam, a town about eleven miles due south from the plaza of the capital.  Between St. Augustin Tlalpam and the city lie the hacienda of San Antonio and the village of Churubusco, and south-west of them is Contreras.  All these points, except St. Augustin Tlalpam, were intrenched and strongly garrisoned.  Contreras is situated on the side of a mountain, near its base, where volcanic rocks are piled in great confusion, reaching nearly to San Antonio.  This made the approach to the city from the south very difficult.

The brigade to which I was attached—­Garland’s, of Worth’s division—­was sent to confront San Antonio, two or three miles from St. Augustin Tlalpam, on the road to Churubusco and the City of Mexico.  The ground on which San Antonio stands is completely in the valley, and the surface of the land is only a little above the level of the lakes, and, except to the south-west, it was cut up by deep ditches filled with water.  To the south-west is the Pedregal—­the volcanic rock before spoken of—­over which cavalry or artillery could not be passed, and infantry would make but poor progress if confronted by an enemy.  From the position occupied by Garland’s brigade, therefore, no movement could be made against the defences of San Antonio except to the front, and by a narrow causeway, over perfectly level ground, every inch of which was commanded by the enemy’s artillery and infantry.  If Contreras, some three miles west and south, should fall into our hands, troops from there could move to the right flank of all the positions held by the enemy between us and the city.  Under these circumstances General Scott directed the holding of the front of the enemy without making an attack until further orders.

On the 18th of August, the day of reaching San Augustin Tlalpam, Garland’s brigade secured a position within easy range of the advanced intrenchments of San Antonio, but where his troops were protected by an artificial embankment that had been thrown up for some other purpose than defense.  General Scott at once set his engineers reconnoitring the works about Contreras, and on the 19th movements were commenced to get troops into positions from which an assault could be made upon the force occupying that place.  The Pedregal on the north and north-east, and the mountain on the south, made the passage by either flank of the enemy’s defences difficult, for their work stood exactly between those natural bulwarks; but a road was completed during the day and night of the 19th, and troops were got to the north and west of the enemy.