That only works when the barbarians are rational enough so that they a). believe that you do in fact have such a device, and b). care whether you have it or not. If, instead, the barbarians are committed to invading you even at the cost of their own lives, or believe that their gods/manifest destiny/force of will/whatever will protect them against any retaliation by you, then your doomsday device is useless.
That only works when the barbarians are rational enough so that they a). believe that you do in fact have such a device, and b). care whether you have it or not. If, instead, the barbarians are committed to invading you even at the cost of their own lives, or believe that their gods/manifest destiny/force of will/whatever will protect them against any retaliation by you, then your doomsday device is useless.