If you assume military control of the other nations you can fight as a single unit. If you don't want to go that way, you can rely on some very though and cooperative AI. I got f'd up as France when Nat. Spain and Germany suddenly pocketed my men with a united front that went from the south to the north.
You should really buy AoD in my opinion. I have played HOI III too, I didn't hate it or so but AoD has the more right feeling of being actually realism and sandbox-fun together.