Four Years War

The Four Years War was a series of conflicts between the Klingon Empire and the United Federation of Planets.

The war started when the Federation colony on Archanis IV was attacked which resulted in the death of 112 Federation citizens. It was discovered later that the Federation protectorate world of Axanar served as a base for a Klingon garrison. This was discovered when Captain Garth of the USS Xenophon discovered a disabled Klingon cruiser inside Federation space. Admiral Kkorhetza of the Klingon garrison declined the opportunity to surrender the garrison to Starfleet which started the war.

(TOS novel: FASA Star Fleet Intelligence - Agent's Orientation Sourcebook)