In Assassin’s Creed Odyssey you can burn War Supplies in Forts to bring down a nation’s power (fortification level) so that you can trigger conquest battles.
War Supplies are always in Forts. First, you’ll want to use your eagle and mark them. They will have a purple icon.
To Burn War Supplies: Use your Torch! Hold D-Pad Right and then aim and throw the torch (L2 + R2 on PS4 / LT + RT on XB1 / on PC hold weapon switch button).
It takes a few seconds for the supplies to burn down. Now repeat for the other ones in the fortress, there’s usually 3 of them in the same location. You have infinite torches, just hold D-Pad Right again to use another.
Bonus Tip: You can use the Torch to set enemies on fire. This is particularly helpful against animals as fire deals them a lot of damage over time and they burn quicker than soldiers.