How about a campaign or mission where there is a war going on between two nations and you are sent in as an allie for one of them. However, when you get there, for whatever reason, the side that you have come to help turns against you. So now instead of fighting along side another force, you are now fighting the original enemy as well as the guys you were suppose to help.
There could be plenty of missions for a campaign where you are suppose to do something simple like ambush a convoy, but while you are trying to set mines or whatever, the 3rd force attacks your position so now you have to try to hold them off AND ambush the convoy, making it that much harder.