well first you need a way to start out the campaign. so force A tries to take over force B's territory (basically just a force on force game)
for example force A wins the first scenario, force B has to counterattack and destroy their documents in a bunker that force A took etc
so lets say force doesn't succeed in the 2nd scenario, now force A has intel and knows key objectives to destroy that will greatly impact force B
to put it simply, who ever wins the scenarios will affect the way the game goes, be sure to have the campaign able to go both ways.
just my thoughts