Following his capture of Fort Henry on February 6, Grant moved his army (later to become the Union's Army of the Tennessee) 12 miles (19 km) overland to Fort Donelson, from February 11 to 13, and conducted several small probing attacks. On February 14, Union gunboats under Flag Officer Andrew H. Foote attempted to reduce the fort with gunfire, but were forced to withdraw after sustaining heavy damage from the fort's water batteries.

On February 15, with the fort surrounded, the Confederates, commanded by Brig. Gen. John B. Floyd, launched a surprise attack, led by his second-in-command, Brig. Gen. Gideon Johnson Pillow, against the right flank of Grant's army. The intention was to open an escape route for retreat to Nashville, Tennessee. Grant was away from the battlefield at the start of the attack, but arrived to rally his men and counterattack. Pillow's attack succeeded in opening the route, but Floyd lost his nerve and ordered his men back to the fort. The following morning, Floyd and Pillow escaped with a small detachment of troops, relinquishing command to Brig. Gen. Simon Bolivar Buckner, who accepted Grant's demand of unconditional surrender later that evening. The battle resulted in virtually all of Kentucky as well as much of Tennessee, including Nashville, falling under Union control. The capture opened the Cumberland River, an important avenue for the invasion of the South. It 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.