From the docking point, run up the dock and climb the first building you come across at the end. From the roof here, turn on your eagle vision and you should be able to spot the French captain patrolling around below you and to the southwest. Drop down ad kill him.
Continue to the southwest until you spot a guard tower. Beneath this, you will be able to find the French poison gas reserve that we need to destroy. Climb up and eliminate the sniper, or sleep dart him so that you can approach the barrels undetected. Once you are clear, either shoot them from a distance, or waltz up and interact with it to ignite it.
Finally, we need to make our way over to the southeast corner of the restricted zone. Here you will find the two executioners that we need to kill hanging out under an open sheltered area. Sneak up and double assassinate them, or take them on in open combat. Once they are dead, the settlement will be all yours.