In the Battle of Antietam Lee's lines had been pressed hard, but had held firm. He decided to remain on the battlefield the next day and hazard a renewal of the battle. However, there was no fighting the next day. Lee was in no condition to fight, and although McClellan had fresh troops available, he was convinced that Lee was planning to attack him. After the a truce to remove the wounded, Lee retreated that night across the river back into Virginia.
Lee's first invasion of the north was over. The campaign had started out with brilliant prospects, but the discovery of Lee's lost orders spurred the usually sluggish McClellan into quick action. Lee was able to capture Harper's Ferry and reunify his army. In the Battle of Sharpsburg or Antietam he beat off uncoordinated Union attacks and McClellan did not continue to press with his superior numbers. Lee retreated across the river, his invasion beaten back. The campaign had turned upon the providential discovery of Special Orders 191. If that event had not happened, the campaign, and perhaps even the war, may well have turned out very differently.