Speak to Chauxnatte (K-10). She will give you the option to decide which fort you want to ride to.
You will start in East Ronfaure (S). Please keep in mind that even though the NPC does not mention it, you are under a strict time limit. Being very familiar with the route you need to take is highly recommended.
Once you've reached the fort head towards the Campaign Arbiter to receive a cut scene.
Bracing for the possibility of prolonged besiegement from the beastmen, work has commenced on increasing the storage capacity of Allied strongholds by way of expanding provision storehouses. Deilver the necessary construction materials by chocobo to one of our nation's areas.