I am not completely clear if the Dux and Civitates can drag each other into battle against their will versus Barbarians. How does this work?Actually, there's no dragging anyone: as long as there is Briton Cooperation (i.e. under every Imperium status except Fragmentation), the Dux and Civitates ALWAYS fight together against barbarians, there is no choice. There are some rules governing the allocation of losses if the two factions cannot agree, and one Briton faction cannot drag against their will units of the other from one space to another, but when there is a battle against barbarians, all Britons stick together before Fragmentation.
Hope this helps :-)