The American army advanced by the way of Ayotla.  The route was the great national road from Vera Cruz to Mexico.  The last fifteen miles of the way was occupied with fortifications, both natural and artificial, and it seemed impossible to advance directly to the gates of the city.  The army was accordingly brought around Lake Chalco, and thence westward to San Augustine.  This place is ten miles from the capital.  The approach now lay along causeways, across marshes and the beds of bygone lakes.  At the further end of each causeway, the Mexicans had built massive gates.  There were almost inaccessible positions at Contreras, San Antonio and Molino del Rey.  Further on toward the city lay the powerful bulwarks of Churubusco and Chapultepec.  The latter was of great strength, and seemed impregnable.  These various outposts were held by Santa Anna with a force of fully thirty thousand Mexicans.

The first assaults of the Americans were made on the nineteenth of August, by Generals Pillow and Twiggs.  The line of communications between Contreras and Santa Anna’s army was cut, and in the darkness of the following night an assault was made by General Persifer F. Smith, who about sunrise carried the place and drove the garrison pell-mell.  This was the first victory of the memorable twentieth of August.

A few hours later, General Worth compelled the evacuation of San Antonio.  This was the second victory.  About the same time, General Pillow advanced on Churubusco, and carried one of the heights.  The position was taken by storm, and the enemy scattered like chaff.  This was the third triumph.  The division of General Twiggs added a fourth victory by storming and holding another height of Churubusco, while the fifth and last was achieved by General Shields and Pierce, who drove back an army of reinforcements under Santa Anna.  The Mexicans were thus forced back into the fortifications of Chapultepec.

On the following morning, the alarm and treachery of the Mexican authorities were both strongly exhibited.  A deputation came out to negotiate; but the intent was merely to gain time for strengthening the defences.  The terms proposed by the Mexicans were preposterous when viewed in the light of the situation.  General Scott, who did not consider his army vanquished, rejected the proposals with scorn.  He, however, rested his men until the seventh of September before renewing hostilities.  On the morning of the eighth, General Worth was thrown forward to take Molino del Rey and Casa de Mata, which were the western defences of Chapultepec.  These places were defended by about fourteen thousand Mexicans; but the Americans, after losing a fourth of their number in the desperate onset, were again victorious.  The batteries were now turned on Chapultepec itself, and on the thirteenth of September that frowning citadel was carried by storm.  This exploit opened an avenue into the city.  Through the San Cosine and Belen gates the conquering army swept resistlessly, and at nightfall the soldiers of the Union were in the suburbs of Mexico.