The Battle of Fort Donelson was fought from February 11 to February 16, 1862, in the Western Theater of the American Civil War. The capture of the fort by Union forces opened the Cumberland River, an important avenue for the invasion of the South. The success elevated Brig. Gen. Ulysses S. Grant from an obscure and largely unproven leader to the rank of major general, earning him the nickname "Unconditional Surrender" Grant in the process (using his first two initials, "U.S.").