Fort Donelson, on the Cumberland River, is…
February 1862 CE
Grant's army marches cross-country in pursuit of Tilghman's men and attempts immediate assaults on the fort from the rear, but they are unsuccessful.
On February 15, the Confederate forces under Brigadier General John B. Floyd attempt to escape and launch a surprise assault against the Union right flank (commanded by Brigadier General John A. McClernand), driving McClernand's division back but not creating the opening they need to slip away.
Grant recovers from this temporary reversal and assaults the weakened Confederate right.
Trapped in the fort and the town of Dover, Tennessee, Confederate Brigadier General Simon B. Buckner surrenders his command of eleven thousand five men and many needed guns and supplies to Grant's demand for "unconditional surrender".
The combined victories at Henry and Donelson are the first significant Union victories in the war, and two major rivers become available for invasions into Tennessee.