[Unicron]

It's also mentioned that the war dragged on for two decades, with the Feds still able to win at least some minor victories even while nearing defeat (Riker mentions the Klingons were trounced at the alternate Archer IV). That suggests the war was based on attrition to some extent, and other powers might have helped one faction or the other even if they didn't directly get involved (which would widen the war, not necessarily desirable).