The Battle of Antietam, fought on September 17, 1862, near Sharpsburg, Maryland, and Antietam Creek, as part of the Maryland Campaign, was the first major battle in the American Civil War to take place on Union soil. It was the bloodiest single-day battle in American history, with about 23,000 casualties on both sides.

Data source: http://www.usna.edu/Users/oceano/pguth/website/kml/antietam.htm