Please open your hymnals to page 18, second paragraph from the top. (Second rulebook print run as posted in the files section)
If a hex contains more than two parties, battles are first resolved among the players' Ships with the sole surviving player then engaging the Ancients or the GCDS in battle.
Then paragraph 7
Each Ship type (Interceptor, Cruiser...) of each player are activated in Initiative order. The Ship type with the highest Initiative goes first. All Initiative ties are resolved in the defender’s favor. The defender is always the player who has an Influence Disc on the hex. In the case of a hex with no disc, or a hex with more than two players, the defender is the player who moved there first.
So for the base game, the Ancient is always the defender.