"McClellan had plenty of reserves waiting outside Sharpsburg, but he never used them. Lee, outnumbered 3-to-1, braced for a new attack all the next day. It never came. On the 18th, Lee and his army slipped back across the Potomac. McClellan could claim a victory, but he could have won the War. Lee’s invasion had been halted, he had suffered terrible losses, but his army had not been destroyed."