The Klingon Civil War was a conflict fought in 2368 between the forces of Klingon Chancellor Gowron and forces loyal to the House of Duras. The war was fought over the Duras claim to the chancellorship, due to perceived Federation influence in the succession process that chose Gowron as the new Chancellor.
The war was won by Gowron's forces, though not without Federation assistance in exposing Romulan support of the Duras forces. (TNG: "Redemption", "Redemption II")
Starfleet crewmembers who served on the ships that blockaded the Klingon-Romulan border to prevent Romulan interference were awarded the Klingon Civil War Blockade Medal.