This affair, like that of Cerro Gordo, was an engagement in which the officers of the engineer corps won special distinction.  In fact, in both cases, tasks which seemed difficult at first sight were made easier for the troops that had to execute them than they would have been on an ordinary field.  The very strength of each of these positions was, by the skill of the engineers, converted into a defence for the assaulting parties while securing their positions for final attack.  All the troops with General Scott in the valley of Mexico, except a part of the division of General Quitman at San Augustin Tlalpam and the brigade of Garland (Worth’s division) at San Antonio, were engaged at the battle of Contreras, or were on their way, in obedience to the orders of their chief, to reinforce those who were engaged.  The assault was made on the morning of the 20th, and in less than half an hour from the sound of the advance the position was in our hands, with many prisoners and large quantities of ordnance and other stores.  The brigade commanded by General Riley was from its position the most conspicuous in the final assault, but all did well, volunteers and regulars.

From the point occupied by Garland’s brigade we could see the progress made at Contreras and the movement of troops toward the flank and rear of the enemy opposing us.  The Mexicans all the way back to the city could see the same thing, and their conduct showed plainly that they did not enjoy the sight.  We moved out at once, and found them gone from our immediate front.  Clarke’s brigade of Worth’s division now moved west over the point of the Pedregal, and after having passed to the north sufficiently to clear San Antonio, turned east and got on the causeway leading to Churubusco and the City of Mexico.  When he approached Churubusco his left, under Colonel Hoffman, attacked a tete-de-pont at that place and brought on an engagement.  About an hour after, Garland was ordered to advance directly along the causeway, and got up in time to take part in the engagement.  San Antonio was found evacuated, the evacuation having probably taken place immediately upon the enemy seeing the stars and stripes waving over Contreras.

The troops that had been engaged at Contreras, and even then on their way to that battle-field, were moved by a causeway west of, and parallel to the one by way of San Antonio and Churubusco.  It was expected by the commanding general that these troops would move north sufficiently far to flank the enemy out of his position at Churubusco, before turning east to reach the San Antonio road, but they did not succeed in this, and Churubusco proved to be about the severest battle fought in the valley of Mexico.  General Scott coming upon the battle-field about this juncture, ordered two brigades, under Shields, to move north and turn the right of the enemy.  This Shields did, but not without hard fighting and heavy loss.  The enemy finally gave way, leaving in our hands prisoners, artillery and small arms.  The balance of the causeway held by the enemy, up to the very gates of the city, fell in like manner.  I recollect at this place that some of the gunners who had stood their ground, were deserters from General Taylor’s army on the Rio Grande.