So, what would be your solution?
My solution is to stop teaching ninjutsu except to a small group of 30 people or so, that must be neutral so they won't be in the side of any country. In case future wars, this small group would stop it.
My solution is to stop teaching ninjutsu except to a small group of 30 people or so, that must be neutral so they won't be in the side of any country. In case future wars, this small group would stop it.