Fort Henry quickly fell when Union gunboats bombarded it. Fort Heiman, intended to reinforce Fort Henry, wasn't yet completed, so the Confederates quickly abandoned it.
Fort Donelson posed more of a challenge. It was well defended with both heavy guns controlling the Cumberland River, and troops and reinforcements defending an outer defense line. Still, Union forces moved to surround the fort. A winter blizzard quickly made conditions miserable. The Confederates, hoping to avoid starvation, attacked Union troops on one end of the line in order to escape to Nashville. Just as the Confederates were advancing and nearly free and clear, a poor command decision ordered them back to the fort. Union troops moved back in to surround the fort and demanded that Southern troops surrender. Ulysses S. Grant would not accept any terms other than unconditional surrender, earning him the name "Unconditional Surrender" Grant. The two top Confederate generals fled the scene by river, leaving General Buckner to surrender to Grant.
The defeat of Fort Donelson opened a door to the South for the Union Army. 13,000 Confederate soldiers were shipped north by river to prisoner of war camps. Many were exchanged for Union prisoners later that year. Grant was promoted to major general after this victory. Grant refused to return escaped slaves to their owners, so freedmen camps were established behind Union lines. 300 slaves wintered at Fort Donelson in 1864.Fort Donelson by the numbers--North and South:
The former Dover Hotel where General Grant accepted General Buckner's unconditional surrender: