The American Civil War significantly affected Tennessee, with every county witnessing combat. During the War, Tennessee was a Confederate state, and the last state to officially secede from the Union to join the Confederacy. Tennessee had been threatening to secede since before the Confederacy was even formed, but didn’t officially do so until after the fall of Fort Sumter when public opinion throughout the state drastically shifted. Tennessee seceded in protest to President Lincoln's April 15 Proclamation calling forth 75,000 members of state militias to suppress the rebellion. Although Tennessee provided a large number of troops for the Confederacy, it would also provide more soldiers for the Union Army than any other state within the Confederacy.

The American Civil War was a civil war in the United States between the Union and the Confederacy, which had been formed by states that had seceded from the Union.