This Day in American Military History

17 September 1862: The bloodiest day in U.S. history—the Battle of Antietam. George B. McClellan’s Army of the Potomac fought Robert E. Lee’s Army of Northern Virginia. The clash near the Maryland town of Sharpsburg left 23,000 Union and Confederate soldiers dead, wounded, or missing in 12 hours of savage fighting. Antietam is generally regarded as a strategic victory for the Union, ending the Confederate Army’s first invasion of the North. It also gave President Abraham Lincoln the confidence to issue a preliminary Emancipation Proclamation just days later.

Brandus, Paul. This Day in U.S. Military History (p. 226). Bernan Press. Kindle Edition.