The Battle of Gettysburg (July 1-3 1863) was by far the battle in the American Civil War that had the most casualties, with over 51,000 wounded, of which 7,863 dead. Gettysburg lasted 3 days, and the battle with the second highest amount of fatalities for the Civil War, the Battle of Spotsylvania Court House, falls far behind with 4,192 deaths. This battle was also fought over 13 days, May 8 through May 21, 1864, so it wasn't nearly as bloody as Gettysburg.
The battle that however claimed the most casualties in a single day, was the Battle of Antietam (or Battle of Sharpsburg), making it the bloodiest battle fought in the Civil War. The Battle of Antientam took place on September 17, 1862 and the total amount of people wounded, dead and missing in action is estimated at 22,717. It was the first major Civil War battle that took place in Union territory.
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