It's the equivalent to walking into someone else's 1 on 1 fight and punching one in the face. Of course you can, but you are still an asshole, and at some point someone is going to teach you a lesson.
Saying it's the responsibility of the province to not be vulnerable is an asinine statement. If a kingdom is in a retal war, they will naturally become weaker than other provinces who are not in conflict. There is no avoiding that.