sentences:  Shortly after McClellan's return to command, General Lee's forces crossed the Potomac River into Maryland, leading to the Battle of Antietam in September 1862.The ensuing Union victory was among the bloodiest in American history, but it enabled Lincoln to announce that he would issue an Emancipation Proclamation in January.Following the battle, McClellan resisted the president's demand that he pursue Lee's retreating and exposed army.