Santa Anna did not concur with Valencia's positioning of his forces, and as Supreme Commander orders him to withdraw to Coyoacan and Churubusco.

[1]: 290  Rather than moving northward that would have taken them to the choke point at El Peñon, which Mexican General Antonio López de Santa Anna had fortified, Scott chose a circuitous southerly route to attack Mexico City.

Santa Anna understood Scott's flanking action, and sought to block alternative routes into the capital.

[1]: 291 General Valencia had not yet faced U.S. troops in battle, and did not consider the spot that Santa Anna had ordered him to at San Ángel to be a defensible position.

Valencia had not expected the U.S. forces to be able to cross the lava field at El Pedregal, considering it impassable terrain and a natural defense.

[1]: 292  On the morning of 19 August, Lee's men met Mexican pickets, which Major William W. Loring's companies cleared at Padierna, only to come under fire from Valencia's 22 pieces of artillery west of El Pedregal and north of Contreras.

Ransom's 9th Infantry and Lt. Col. Milledge L. Bonham's 12th moved forward on the American right to within 200 yards of Valencia's camp by nightfall.

He moved with Brigadier General Francisco Pérez's 3,000 man brigade to San Ángel, just north of the fighting.

[9] During the night, Lt. Zealous Bates Tower discovered a ravine running southwest from San Jerónimo to the rear of Valencia's camp, which Smith planned to use for a dawn attack the next morning.

[1]: 295  Two of the cannons the American forces captured had been lost by Captain John P. O'Brien in bloody fighting at the Battle of Buena Vista in February 1847.

[1]: 295  The contemporary newspaper dispatch by George Wilkins Kendall names the generals: Miguel Blanco de Estrada, Manuel García Pueblita, N. Mendoza, and former President of Mexico and brigadier general José Mariano Salas, called "the notorious" for organizing the Guerrillas of Vengeance against the Americans.

[12] "Valencia's repeated refusal to follow orders had led to the dissolution of Mexico's most experienced and disciplined regiments and exposed the rest of the army to the same possibility.

[1]: 305 Santa Anna issued his version of events of the battle on 23 August 1847, which appeared in English translation in the New Orleans Daily Picayune on 9 September.

On 20 August 1847, General Scott made a speech from which the first sixteen words have become important to the Regiment of Mounted Riflemen.

The regiment was bloodied and exhausted from the fierce fighting at Contreras, but even so, each man stood at attention as Scott approached.