Santa Anna had 36,000 men waiting for Scott when he neared Mexico City in August 1847, so Scott turned south. Santa Anna redeployed with General Valencia near Contreras.

The Americans faced a massive lava field near Contreras, but Robert E. Lee built a road through it. Early August 20, General Gideon Pillow's army attacked from the north, while another army launched a sneak attack from behind. Among them was Captain Allen Wood's Company C (12th Infantry), who wrote of his Arkansans, “We all hastened as rapidly as the ground would permit, forded the stream, and poured into their dismayed ranks a most destructive fire." Valencia's troops fled.