The other spots must still be held for this task to appear and be able to be completed-it will cancel if a prerequisite location is lost at any point in the process. Once the allied faction is strong enough you'll get a new task to capture a location currently held by the enemy. You don't always have to take part in a battle if the ally bars are stronger than the enemy bars there is a greater chance of them holding that location without your help.
Destroying enemy groups and stopping attacks will weaken the enemy overall and protecting allied groups will prevent their strength being reduced. The faction strength bars show how the war is going. You don't have to join a faction but you can get a lot of goodies after the final point is captured (e.g.
