Maybe Fear.
If one side can't get fear into the other side's soldiers, often it can be put into the general populace.
It was also used in ancient times, promoted by displaying the size of an army, heralds, written messages or dead bodies hurled over barriers.
Terrorists use this tactic.
The greatest enemy we have to fear, is fear itself.
Must a weapon be a "thing." or can it be any action, thing or influence?