This Day in U.S. History:

1862: Ulysses S. Grant earned the nickname “Unconditional Surrender” Grant after the Battle of Fort Donelson. The battle waged around a Confederate fort near the Kentucky-Tennessee border, proved to be a significant Union victory—the Army of Central Kentucky had almost 14,000 casualties. Grant, already a brigadier general, would be promoted to major general.

Brandus, Paul. This Day in U.S. Military History (p. 49). Bernan Press. Kindle Edition.