At the start of the Battle of Gettysburg, July 1, 1863, Doubleday's division was the second infantry division on the field to reinforce the cavalry division of Brigadier General John Buford. When his corps commander, Major General John F. Reynolds, was killed very early in the fighting, Doubleday found himself in command of the corps at 10:50 am. His men fought well in the morning, putting up a stout resistance, but as overwhelming Confederate forces massed against them, their line eventually broke and they retreated back through the town of Gettysburg to the relative safety of Cemetery Hill south of town. It was Doubleday's finest performance during the war, five hours leading 9,500 men against ten Confederate brigades that numbered more than 16,000. Seven of those brigades sustained casualties that ranged from 35 to 50 percent, indicating the ferocity of the Union defense. On Cemetery Hill, however, the I Corps could muster only a third of its men as effective for duty, and the corps was essentially destroyed as a combat force for the rest of the battle; it would be decommissioned in March 1864, its surviving units consolidated into other corps.