Greene was again temporarily elevated to command of his division, now designated part of the XII Corps of the Army of the Potomac, at the Battle of Antietam. His division's three brigades were led by junior officers who had survived Cedar Mountain. Even though XII Corps commander Brig. Gen. Joseph K. Mansfield was killed shortly after the fighting began, Greene led a crushing attack against the Confederates near the Dunker Church, achieving the farthest penetration of Maj. Gen. Stonewall Jackson's lines than any Union unit. Under immense pressure, Greene held his small division (only 1,727 men engaged at the start of the day) in advance of the rest of the army for four hours, but eventually withdrew after suffering heavy losses. While the division was posted to Harpers Ferry, Greene took a three-week sick leave. Maj. Gen. Oliver O. Howard speculated that Greene, like many of his fellow officers, was sickened by the stench of dead and wounded at Antietam. When he returned, there was a new division commander, Brig. Gen. Geary. Greene was disgruntled that Geary, with only a few days seniority over him, was selected for the post; Geary had been wounded at Cedar Mountain and his combat record was not as good, but his political connections and a sentiment that a wounded officer should not be set back in his career unnecessarily, gave him the nod.