Short answers, pulled from the story.
The Battle of Fort Donelson was fought from the 11th to the 16th of February, 1862, in the Western Theater of the American Civil War. The decisive surrender occurred on the morning of the 16th of February 1862.
When Confederate General Simon Bolivar Buckner requested terms of surrender, Grant replied that no terms except unconditional and immediate surrender could be accepted. The initials U.S. in Grant's name aligned neatly with the phrase, and the nickname stuck for the rest of the war.
Brigadier General John B. Floyd feared he would be indicted for corruption from his time as Secretary of War under President James Buchanan and escaped by steamer with his two Virginia regiments. Brigadier General Gideon Pillow also feared Northern reprisals and slipped away by small boat. Nathan Bedford Forrest refused to surrender and led about seven hundred cavalrymen out through the icy waters of Lick Creek.
More than 12,000 Confederate soldiers were captured at Fort Donelson, along with 48 artillery pieces and much of the army's equipment. Total Confederate losses including killed and wounded reached 13,846. More than 7,000 prisoners were transported to Camp Douglas in Chicago, Camp Morton in Indianapolis, and other prison camps in the North.
Flag Officer Andrew H. Foote's ironclad flotilla attacked the fort's river batteries on the 14th of February 1862, and was badly damaged. USS St. Louis was hit 59 times and her wheelhouse was shot away; USS Louisville was disabled; USS Pittsburg began taking on water. Foote himself was wounded. Eight Union sailors were killed and 44 were wounded, while the Confederate water batteries lost no men in the engagement.
The capture opened the Cumberland River to Union invasion and led directly to the fall of Nashville on the 23rd of February 1862, making it the first Confederate state capital taken by Union forces. Columbus, Kentucky was evacuated on March 2, and most of Tennessee along with all of Kentucky came under Union control. Close to a third of Albert Sidney Johnston's entire Confederate force was captured, weakening his army ahead of the Battle of Shiloh.