The Battle of Antietam The Battle of Antietam was one of the bloodiest battles in the Civil War. It took place on September 17, 1862 in Maryland. It was at once the climax and the end of the first Confederate invasion of the North. It happened between the U.S. Army of the Potomac under Gen. George B. McClellan and the Confederate Army of Northern Virginia under Gen. Robert E. Lee. The battle began early in the morning at about 6:00 A.M. and lasted throughout the day. There is no general plan of attack because McClellan had none. The battle resolved itself into 5 separate, practically unrelated asaults conducted by t