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476 He therefore resolved to turn this position by crossing the pedregal and thus disconcert the garrisons by a rear movement, opening at the same time the way toward the Chapultepec line, as the weakest. The task of making a road was assigned to Pillow's division, guarded by Twiggs, the former being authorized to join forces and drive Valencia from his position if he proved troublesome. The latter, who had already feebly contested the ground with the reconnoitring party, was not slow in opening fire, and Pillow proving equally prompt in accepting the challenge, the conflict began early in the afternoon of August 19th. Valencia's skirmishers were driven from the pedregal and beyond the rancho Padierna into the camp, and two light batteries were planted to cover an assault by Smith's brigade. So murderous, however, was the fire from the heavier metal of the intrenchments that the batteries were practically silenced, and the brigade had to fall back for shelter. Another brigade was sent to its aid; but now occurred a diversion of which the Mexicans took advantage for a spirited sally, during which they recaptured the rancho, although holding it only for a time.

The diversion was caused by, the approach of Santa Anna, on the road from San Ángel, at the instance of the insubordinate Valencia, with a force of not less than 6,000. Anticipating some such movement, Pillow had, at the opening of the conflict, despatched Riley's brigade to occupy San Gerónimo, a village situated midst fields and orchards on a small plateau one mile north-west of the intrenchments. His object was also to operate on the flank and rear of the latter; and this soon occupied his exclusive attention,