The Battle of Fort Donelson was fought from February 11–16, 1862, in the Western Theater of the American Civil War. The Union capture of the Confederate fort near the Tennessee–Kentucky border opened the Cumberland River, an important avenue for the invasion of the South. The Union's success also elevated Brig. Gen. Ulysses S. Grant from an obscure and largely unproven leader to the rank of major general, and earned him the nickname of "Unconditional Surrender" Grant.