On this day 163 years ago, on September 17, 1862 (August 24, 1862 lunar calendar), the Battle of Antietam in the American Civil War broke out. On September 18, 1862, the Battle of Antietam in the American Civil War broke out. McClellan's 87,000-man Army of the Potomac met Robert Lee's Army of Northern Virginia (40,000) crossing the Potomac River. On that day, the Union casualties were 12,401 (of which 2,108 were killed in action), and the Confederate casualties were 10,316 (of which 1,546 were killed in action), making it the deadliest day of the Civil War. Comment: Robert Lee retreated after being heavily wounded, while McClellan failed to capture the fighter jets and annihilate the main Confederate army.