Details of the battle:
Grant had about 27,000 men with naval support; the Confederate garrison was roughly 15,000.
Union losses were about 2,250 while the Confederates had about that many fall, and roughly 12,000 surrender.
After capturing Fort Henry on February 6, 1862, Grant advanced cross-country to invest Fort Donelson. The original garrison of the two forts was about 2,500 men, and Albert Johnston had dispatched about 12,000 reinforcements from Bowling Green, KY, under John Floyd to bolster the defense. A few men also arrived from Columbus, the western end of the Confederate defensive line. Grant had wanted to move fast, to prevent reinforcements arriving at all, but wretched weather (rain before and during his operations ruined the roads) delayed him and the Confederate troops arrived safely.
On the 13th the third Union division arrived (Lew Wallace’s) and Grant had his cordon of roughly 25,000 men. The next day Foote opened his naval attack. He had four ironclads and two wooden gunboats, and he led with the ironclads. St Louis (flagship) and Louisville had their steering gear shot away, the other two ironclads had holes punched through their thinner deck armor. The wooden ships didn’t risk themselves much. The Confederates didn’t lose a gun or a man killed.
The Confederates didn’t think about supplying themselves by water; it was unconventional, and they were cut off by land. They also thought Grant was stronger than he really was, and being continually reinforced. They worried that Foote would turn up with another fleet. In a council of war on the night of February 14, they decided to try and break out.
The plan was to reinforce the left (Pillow) with Buckner’s men from the right. Pillow would lead the attack and clear the road to Charlotte and Nashville. Buckner’s men would keep the road open while the rest of the garrison was evacuated, and everybody would join the main army in central Tennessee. It almost happened. Pillow, with Buckner joining the attack, broke the Union line in late morning. They captured 300 prisoners, about 5,000 rifles, and an artillery battery. Indecision then lost the victory. Pillow was cautious, Buckner bold, and Floyd foolish. After hemming and hawing Floyd ordered his (victorious) men to return to the trenches. Meanwhile Grant returned to the field. He’d been conferring with Foote (wounded, so the conference had to be on the gunboat) and returned in time to rescue the battle.
He ordered his reserve into action, attacking the Fort rather than the victorious Confederate left. The deploying attack was what drew the Confederates back to their trenches, but Buckner’s men didn’t arrive in time. They held the Union to only limited gains, but the Confederate’s main defensive line was broken. They had broken the Union line, but snatched defeat from the jaws of victory. Grant also rallied his right, and reoccupied most of the ground lost in the morning, so the day closed with a near-total Union advantage.
On February 16, 1862, the 12,000-man garrison surrendered. Buckner had enquired about terms and Grant uncompromisingly replied “No terms except an unconditional and immediate surrender can be accepted.” Buckner fumed that it was “ungenerous and unchivalrous” but it was warlike. With a demoralized command he had no choice.
This was a major victory for Grant and a catastrophe for the South. It ensured that Kentucky would stay in the Union and opened up central Tennessee to a Northern advance along the Tennessee and Cumberland rivers. Grant received a promotion to major general for his victory and attained stature in the Western Theater, earning the nom de guerre “Unconditional Surrender.”