In scenario 14.1.09 the Peloponesian League has a base and two land units in the Hellespont (13), a naval theater. How can the Athenians ever mount an attack there as it calls for a mandatory naval battle but there are no enemy naval units present? Can they mount a naval attack at the base, only?
Bob has essentially answered your question. A battle for the attacker can theoretically consist of one Strategos and a flipped card. What won't happen is no units will be eliminated since you do not have any naval or land units. Also remember a base is a 2 strength land and naval unit. So it is very possible for the Athenians in your example to have committed some number of Strategos plus the 2 naval strength of the base plus the card flip.
I hope that helps,