As stated in the Star Trek Encyclopedia (1st ed., pg. 18), "it was not established who the opponent was in Garth's victory. It has been speculated that it might have been the Romulans, although the history implied by TOS: "Balance of Terror" indicates that there was no Federation contacts with the Romulans during that time frame." Additionally, based on Kirk's comments, "the Axanar battle apparently had something to do with holding the Federation together." According to the FASA source book The Four Years War, the battle was fought between the Federation and the Klingon Empire.