Complex sentence: The defence of Karachi Harbour was paramount to the Pakistani High Command and it was heavily defended against any air or naval strikes, including cover from strike aircraft based at two airfields in the area.
Split into simpler sentences: The defence of Karachi Harbour was predominant to the Pakistani High Command and it was heavily defended against any air or naval strikes. The port's airspace was secured by the strike aircraft based at two airfields in the area.