Reinforcements were arriving on both sides, and by 10:30 a.m. Robert E. Lee sent his final reserve division—some 3,400 men under Maj. Gen. Richard H. Anderson—to bolster Hill's line and extend it to the right, preparing an attack that would envelop French's left flank. But at the same time, the 4,000 men of Maj. Gen. Israel B. Richardson's division arrived on French's left. This was the last of Sumner's three divisions, which had been held up in the rear by McClellan as he organized his reserve forces.^[23] Richardson's fresh troops struck the first blow.

Leading off the fourth attack of the day against the sunken road was the Irish Brigade of Brig. Gen. Thomas F. Meagher. As they advanced with emerald green flags snapping in the breeze, a regimental chaplain, Father William Corby, rode back and forth across the front of the formation shouting words of conditional absolution prescribed by the Roman Catholic Church for those who were about to die. (Corby would later perform a similar service at Gettysburg in 1863.) The mostly Irish immigrants lost 540 men to heavy volleys before they were ordered to withdraw.^[24]