What was the bloodiest single-day battle of the Civil War?

The Battle of Antietam was the bloodiest single-day battle of the Civil War. Fought on September 17, 1862, near Sharpsburg, Maryland, it produced roughly 22,700 combined Union and Confederate casualties in one day.

The battle began when Union forces under Major General George B. McClellan attacked Confederate General Robert E. Lee's Army of Northern Virginia. Fighting raged across locations including Miller's Cornfield, the Sunken Road—later called Bloody Lane—and Burnside Bridge. Neither army achieved a decisive tactical victory, but Lee withdrew across the Potomac River, making the result strategically favorable to the Union.

Antietam also had major political consequences. The battle stopped Lee's first invasion of the North and gave President Abraham Lincoln the opportunity to issue the preliminary Emancipation Proclamation five days later. Gettysburg caused more total casualties over three days, but Antietam remains the war's bloodiest single day. It is also often called the bloodiest day in American history.