September 12, 1861
- Siege of Lexington, Missouri begins
- September 12 to 15—Battle of Cheat Mountain, Virginia (now West Virginia)
- Lee, with 30,000 men under his command, expected to fight a force led by the Unionist General Rosecrans at Meadow Bridge, western Virginia. However, at the last moment Rosecans changed his direction of movement and engaged a Confederate force at Cheat Mountain, comprehensively defeating them. Union losses were 9 dead and 12 wounded while the Confederates lost nearly 100 men.
A Chronological History of the Civil War in America1
- Col. John A. Washington (rebel), former proprietor of Mount Vernon, killed near Elk Water, Va., while reconnoitering.
- Mayor Berrett, of Washington, took the oath and was released from Fort Lafayette.
- The provost-marshal of St. Louis issued a proclamation manumitting two slaves, the property of a secessionist.
- Battle of Cheat Mountain, Va.; rebels defeated.
- A Chronological History of the Civil War in America by Richard Swainson Fisher, New York, Johnson and Ward, 1863