Are you going to the fort or the marker on the map? The reason I ask is that you have to meet the other soldiers to begin the attack.
If you are doing it right but it is a bug you can try resetting the quest or completing it with the console command.
i "met" the soldiers (got the marker without any soldiers nearby) and the quest went on with -> clear the fort 100 % remaining -> but neither were there any of my soldier nor enemies in or outside the fort to fight against.
the problem is my platform is ps3 ....