Confederate reinforcements arrived just after 7 a.m. The divisions under McLaws and Richard H. Anderson arrived following a night march from Harpers Ferry. Around 7:15, General Lee moved George T. Anderson's Georgia brigade from the right flank of the army to aid Jackson. At 7 a.m., Hood's division of 2,300 men advanced through the West Woods and pushed the Union troops back through the Cornfield again. The Texans attacked with particular ferocity because as they were called from their reserve position they were forced to interrupt the first hot breakfast they had had in days. They were aided by three brigades of D.H. Hill's division arriving from the Mumma Farm, southeast of the Cornfield, and by Jubal Early's brigade, pushing through the West Woods from the Nicodemus Farm, where they had been supporting Jeb Stuart's horse artillery. Some officers of the Iron Brigade rallied men around the artillery pieces of Battery B, 4th U.S. Artillery, and Gibbon himself saw to it that his previous unit did not lose a single caisson.^[18] Hood's men bore the brunt of the fighting, however, and paid a heavy price—60% casualties—but they were able to prevent the defensive line from crumbling and held off the I Corps.

Hooker's men had also paid heavily but without achieving their objectives. After two hours and 2,500 casualties, they were back where they started. The Cornfield, an area about 250 yards (230 m) deep and 400 yards (400 m) wide, was a scene of indescribable destruction. It was estimated that the Cornfield changed hands no fewer than 15 times in the course of the morning.^[19] Maj. Rufus Dawes, who assumed command of Iron Brigade's 6th Wisconsin Regiment during the battle, later compared the fighting around the Hagerstown Turnpike with the stone wall at Fredericksburg, Spotsylvania's "Bloody Angle", and the slaughter pen of Cold Harbor, insisting that "the Antietam Turnpike surpassed them all in manifest evidence of slaughter."^[20] Hooker called for support from the 7,200 men of Mansfield's XII Corps.

Half of Mansfield's men were raw recruits, and Mansfield was also inexperienced, having taken command only two days before. Although he was a veteran of 40 years' service, he had never led large numbers of soldiers in combat. Concerned that his men would bolt under fire, he marched them in a formation that was known as "column of companies, closed in mass," a bunched-up formation in which a regiment was arrayed ten ranks deep instead of the normal two. As his men entered the East Woods, they presented an excellent artillery target, "almost as good a target as a barn." Mansfield himself was shot in the chest and died the next day.

The new recruits of Mansfield's 1st Division made no progress against Hood's line, which was reinforced by brigades of D. H. Hill's division under Colquitt and McRae. The 2nd Division of the XII Corps, under George Sears Greene, however, broke through McRae's men, who fled under the mistaken belief that they were about to be trapped by a flanking attack. This breach of the line forced Hood and his men, outnumbered, to regroup in the West Woods, where they had started the day. Greene was able to reach the Dunker Church, Hooker's original objective, and drove off Stephen Lee's batteries. Federal forces held most of the ground to the east of the turnpike.