The answer was the Battle of Antietam

Map of the Battle of Antietam — Map: Citypeek, CC BY 3.0, via Wikimedia Commons.

Year: 1862
Where: Washington County,, United States
War: American Civil War
Sides: United States and Union vs Confederate States of America
Troops: 84,278
Casualties: about 22,726

The Battle of Antietam ( an-TEE-təm), also called the Battle of Sharpsburg (particularly in the Southern United States) took place during the American Civil War on September 17, 1862, near Antietam Creek and Sharpsburg, Maryland, between the Army of Northern Virginia, led by Confederate General Robert E. Lee, and the Army of the Potomac, led by Union Major General George B. McClellan. Part of the Maryland Campaign, it was the first field army–level engagement in the Eastern Theater of the American Civil War to take place on Union soil.

From Wikipedia, under CC BY-SA.

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