During the U.S. Civil War (1861–1865), a Union Army post stood in Milan. The Union cause was supported by four Union volunteer infantry regiments, two Union cavalry volunteer regiments, two Missouri Militia units, one provisional militia unit, and a large unit of Sullivan County Home Guards. The Confederate side was supported by four units of Missouri State Guard infantrymen. Soldiers from Sullivan County fought at the Battle of Shiloh, Battle of Atlanta, Battle of Jonesborough, and other major engagements throughout the war. Military skirmishes within the county were mostly confined to bushwhackers.