Virtually nothing. He, and the Army of the Potomac, were completely shocked by the bloodletting of the previous day.

Even so, if McClellan had given the order to attack, the chances are excellent that the Civil War would have been won that day. Lee had, at that moment, roughly 30,000 men who could still stand and fight, and not one of them was truly fresh: all had been engaged. McClellan, who received reinforcements the previous night, had close to 70,000 men fit for combat, of whom almost 30,000 were completely fresh: they had not yet been engaged. By standing where he was, McClellan allowed the single most golden opportunity of the war escape him.