The Battle of Fort Donelson is 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 opena the Cumberland River, an important avenue for the invasion of the South.

The Union's success also elevates Brig. Gen. Ulysses S. Grant from an obscure and largely unproven leader to the rank of major general, and earns him the nickname of "Unconditional Surrender" Grant.

Grant moves his army twelve miles (nineteen kilometers) overland to Fort Donelson from February 11 to 13 and conducts several small probing attacks. (Although the name is not yet in use, the troops serving under Grant are the nucleus of the Union's Army of the Tennessee.)

On February 14, Union gunboats under Flag Officer Andrew H. Foote attempt o reduce the fort with gunfire, but are forced to withdraw after sustaining heavy damage from Fort Donelson's water batteries.

On February 15, with the fort surrounded, the Confederates, commanded by Brig. Gen. John B. Floyd, launch a surprise attack against the right flank of Grant's army in an attempt to open an escape route to Nashville, Tennessee.

Grant, who is away from the battlefield at the start of the attack, arrives to rally his men and counterattack.

Despite achieving partial success and opening the way for a retreat, Floyd loses his nerve and orders his men back to the fort.

The following morning, Floyd and his second-in-command, Brig. Gen. Gideon Johnson Pillow, escape with a small detachment of troops, relinquishing command to Brig. Gen. Simon Bolivar Buckner, who accepts Grant's terms of unconditional surrender later in the day.