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. This is also highly unlikely, as the history implied by TOS: "Errand of Mercy" indicates that the Federation had been involved in a strictly "cold" war with the Klingons during that time. In addition, Garth's reaction to the fact that Kirk seems to have successfully made peace with the Federation's opponents in the battle would exclude the Klingons as a potential enemy.