Worth’s division, now preceded by Harney’s cavalry, moved from San Augustin on the main road toward the City of Mexico.  These were followed by the other divisions of the army.  On this route was situated the pedregal, which is a field of volcanic rock of very uneven surface.  It is between the roads leading to the capital from San Augustin and Padierna.  A reconnoissance of the pedregal was made by Lieutenants Robert E. Lee and Pierre G.T.  Beauregard, who reported that there was a passage for wagons of only a mile, and the remainder might be crossed by infantry, carefully picking the way.  The enemy were in position beyond the pedregal with considerable artillery.

General Scott, on the night of the 18th, ordered a movement in the direction of Padierna.  Worth was ordered to cover San Antonio, Quitman to hold San Augustin, and Pillow to march over the pedregal, while Twiggs was to cover and support Pillow’s movement.  On the morning of this movement the Mexican General Blanco was ordered to construct batteries, and General Mejia to take position on the Pelon Cuauhtitlan to command the expected movements of the American army.  General Santa Anna wrote from San Antonio through the Minister of War to General Valencia, at San Angel:  “The general in chief directs me to say to your Excellency that the enemy having now [August 18th, 3 P.M.] taken up a position on our left in front of San Antonio with a part of his forces, it is clear that to-morrow at the latest he will undertake the attack of this fortification, although it appears there is a movement going on at the same time on our right.  His Excellency therefore directs you at daylight to-morrow morning to fall back with your forces to Coyoacan, and send forward your artillery to the fort and the tete-de-pont at Churubusco.”

General Valencia declined to obey this order, giving his reason as follows:  “I should like much to be able to obey this order, but, in view of present circumstances, my conscience as a military man and my patriotism will not permit me.  I believe the national cause will be lost if I should abandon these positions and the road leading from San Augustin through Padierna to these points.  To me it is as clear as the light of day that the enemy will undertake his attack, if not to-morrow, the day after, and that he desires to make two attacks at the same time, the one true and the other false, and that, should he find at the commencement of his movements one of the points of attack abandoned, as this, for instance, he will pass by this route with all his forces, and thus be enabled to assail our flank and turn our rear; or, if he prefer it, he may pass on without obstruction to the City of Mexico.”