February 13, 1862. Eleven-year-old Willie Lincoln had had a mysterious fever for about a week when, on February 13, his eight-year-old brother, Tad, became sick, too. Doctors assured the parents that there was no cause for alarm, but the parents could not help but remember the death of their second child, three-year-old Edward "Eddie" Baker Lincoln, twelve years earlier. The President spent hours at Willie's bedside, stroking his hair and comforting him.

Simon Bolivar Buckner, Library of Congress

On February 16, ten days after, Grant captured the more important Fort Donelson, on the Cumberland River. At Fort Donelson, when Grant's former friend and West Point classmate, Confederate General Simon Buckner, wrote Grant to negotiate terms of surrender, Grant replied, "No terms except an unconditional and immediate surrender can be accepted." Hence Ulysses S. Grant's nickname "Unconditional Surrender" Grant. Grant's response was an early clue that the Civil War was not going to be a typical historical European war.

Lincoln immediately promoted Grant to major general.

Ulysses S. Grant, Mathew Brady Studios, Library of Congress

The fall of Fort Donelson, and the surrender of 12,000 Confederate soldiers, was the largest blow that the Confederacy had been dealt to date; it buoyed the North's -- and Lincoln's -- spirits, and assured that Kentucky would stay in the Union. Moreover, war is like chess: controlling the board is critical to eventual victory. The Confederate defeat at Fort Donelson moved Union forces 200 miles into enemy territory and left all of western Tennessee open to federal occupation and control.