The capture of Fort Donelson was a pivotal moment in the American Civil War, occurring when Union forces under Major General Ulysses S. Grant laid siege to the Confederate fortification in Tennessee. After a series of battles beginning on February 12, Grant's troops surrounded the fort, cutting off supplies and reinforcements. The siege culminated in an overwhelming assault, which forced the Confederate defenders into surrendering by February 16. General John B. Floyd, who was in command of the Confederate forces, attempted to escape but ultimately relinquished control.

The operation at Fort Donelson revealed Grant's strategic acumen and his willingness to take calculated risks on the battlefield. His forces utilized rapid movement and coordinated attacks, demonstrating a level of initiative that had been somewhat lacking in earlier Union campaigns. The successful capture of the fort opened up the Cumberland River for Union navigation, which became critical for future operations in the western theater of the war. Additionally, it facilitated the advance into Tennessee, paving the way for further Union offensives.

The victory at Fort Donelson also had significant psychological implications. It marked the first major Union victory of the war and provided a boost to Northern morale at a time when early casualties had created a sense of uncertainty about the Union's prospects. It transformed Ulysses S. Grant into a national hero and helped consolidate his reputation, eventually leading to his promotion to Major General. The campaign set the stage for subsequent battles, ultimately contributing to the Union's overall strategy of seizing control over crucial southern territories and waterways.