Gen. Richardson personally dispatched the brigade of Brig. Gen. John C. Caldwell into battle around noon (after being told that Caldwell was in the rear, behind a haystack), and finally the tide turned. Anderson's Confederate division had been little help to the defenders after Gen. Anderson was wounded early in the fighting. Other key leaders were lost as well, including George B. Anderson and Col. John B. Gordon of the 6th Alabama. These losses contributed directly to the confusion of the following events.

As Caldwell's brigade advanced around the right flank of the Confederates, Col. Francis C. Barlow and 350 men of the 61st and 64th New York saw a weak point in the line and seized a knoll commanding the sunken road. This allowed them to get enfilade fire into the Confederate line, turning it into a deadly trap. In attempting to wheel around to meet this threat, a command from Rodes was misunderstood by Lt. Col. James N. Lightfoot, who had succeeded the unconscious John Gordon. Lightfoot ordered his men to about-face and march away, an order that all five regiments of the brigade thought applied to them as well. Confederate troops streamed toward Sharpsburg, their line lost.

Richardson's men were in hot pursuit when massed artillery hastily assembled by Gen. Longstreet drove them back. A counterattack with 200 men led by D.H. Hill got around the Federal left flank near the sunken road, and although they were driven back by a fierce charge of the 5th New Hampshire, this stemmed the collapse of the center. Reluctantly, Richardson ordered his division to fall back to north of the ridge facing the sunken road. His division lost about 1,000 men. Col. Barlow was severely wounded, and Richardson mortally wounded. Winfield S. Hancock assumed division command. Although Hancock would have an excellent future reputation as an aggressive division and corps commander, the unexpected change of command sapped the momentum of the Federal advance.^[27]

The carnage from 9:30 a.m. to 1:00 p.m. on the sunken road gave it the name Bloody Lane, leaving about 5,600 casualties (Union 3,000, Confederate 2,600) along the 800-yard (700 m) road. And yet, a great opportunity presented itself. If this broken sector of the Confederate line were exploited, Lee's army would be divided in half and possibly defeated. There were ample forces available to do so. There was a reserve of 3,500 cavalry and the 10,300 infantrymen of Gen. Porter's V Corps, waiting near the middle bridge, a mile away. The VI Corps, under Maj. Gen. William B. Franklin, had just arrived with 12,000 men. Franklin was ready to exploit this breakthrough, but Sumner, the senior corps commander, ordered him not to advance. Franklin appealed to McClellan, who left his headquarters in the rear to hear both arguments but backed Sumner's decision, ordering Franklin and Hancock to hold their positions.^[28]