The state of Tennessee is rooted in the Watauga Association, a 1772 frontier pact generally regarded as the first constitutional government west of the Appalachians.What is now Tennessee was initially part of North Carolina, and later part of the Southwest Territory. Tennessee was admitted to the Union as the 16th state on June 1, 1796. Tennessee was the last state to leave the Union and join the Confederacy at the outbreak of the U.S. Civil War in 1861 and the first state to be readmitted to the Union at the end of the war.
Tennessee furnished more soldiers for the Confederate Army than any other state, and more soldiers for the Union Army than any other Southern state. In the 20th century, Tennessee transitioned from an agrarian economy to a more diversified economy, aided at times by federal entities such as the Tennessee Valley Authority. In the early 1940s, the city of Oak Ridge was established to house the Manhattan Project's uranium enrichment facilities, helping to build the world's first atomic bomb.