The Battle of Antietam The Battle of Antietam ( or Sharpsburg) on September 17, 1862, climaxed the first of Confederate General Robert E. Lee’s two attempts to carry the war into the north. About 40,000 southerners were against the 87,000- man Federal Army of the Potomac under General George McClellan. When the fighting had ended, the course of the American Civil War had greatly altered. After his great victory at Manassas in August, Lee had marched his Army of Northern Virginia into Maryland, hoping to find vitally needed men and supplies. McClellan followed, first to Frederick ( where through rare good fortune a copy of the Confederate battle plan, Lee