You must be joking as this kind of war would be WW3 and the end of life. We need to sanction the shit out of the people closest to the problem and get the UN out there to investigate.
I have no doubt it would be carnage unlike the world has ever seen. But honestly the real joke is expecting the UN to be able to do anything meaningful. This is an organization that is so politically impotent they can't even stop genocides in places like Sub saharan africa, like they will be able to do anything against a premier world power that sits on the permanent security council. No one likes it but war is a serious answer, calling for UN investigations, well you might as well throw a bottle in the ocean and hope someone in a landlocked country gets your message.