Having already ceded Kentucky to Federal control, the failures at Henry and Donelson were serious setbacks that forced Johnston to withdraw deeper into Tennessee . That movement resulted in the forfeiture of invaluable territory and led to the hasty evacuation of Nashville without even so much as a fight. Abandoning Nashville, her factories and stores, and the city’s 17,000 citizens was an industrial and psychological loss from which the South never fully recovered. Nashville forever remained the lost jewel of the Confederacy, and the dream of regaining the city would die hard.
Jacobson, Eric A. (2013-11-01). For Cause and Country: A Study of the Affair at Spring Hill & the Battle of Franklin (Kindle Location 302). O’More Publishing. Kindle Edition.
By moving into western Kentucky and Tennessee on the rivers, rather than marching overland through eastern Kentucky and Tennessee, a Federal army could turn the Confederacy’s uppermost flank and easily peel open the upper South like ripe fruit. “Move up the Cumberland and Tennessee, making Nashville the first objective point,” Halleck recommended. “This line of the Cumberland or Tennessee is the great central line of the Western theater of war, with… two good navigable rivers extending far into the interior of the theater of operations.”
Guelzo, Allen C. (2012-04-20). Fateful Lightning: A New History of the Civil War and Reconstruction (p. 195). Oxford University Press. Kindle Edition.
The capture of forts Henry and Donelson was enormously important and might almost be called the first turning point of the war. Up until that time, the Confederacy had had things pretty much its own way, but it never fully recovered from the loss of the forts. The twin February victories gave the Union control of the Tennessee and Cumberland rivers deep into what had been Rebel-controlled territory. Union gunboats, transports, and armies could range up the Cumberland all the way to Nashville and up the Tennessee all the way into northern Mississippi and Alabama. The Rebels could no longer hold positions along the Mississippi in the states of Tennessee or Kentucky because Union control of the Tennessee guaranteed the defenders would easily be turned and possibly trapped. Half the state of Tennessee passed to Union control, and the partial Confederate grasp on the state of Kentucky was gone. Within weeks, Union troops would be poised on the edge of the Deep South states.
Woodworth, Steven E. (2011-04-16). This Great Struggle: America’s Civil War (Kindle Locations 1717-1723). Rowman & Littlefield. Kindle Edition.
Strategically, the victories at Forts Henry and Donelson opened the Tennessee and Cumberland rivers as avenues of irresistible Union advance, completely unraveling Confederate defenses from the Mississippi to Nashville and southward to the southern boundary of Tennessee. Seven days after the fall of Donelson, the Confederates evacuated Nashville.
Woodworth, Steven E. (2007-12-18). Nothing but Victory: The Army of the Tennessee, 1861-1865 (Vintage Civil War Library) (p. 119). Knopf Doubleday Publishing Group. Kindle Edition.