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I don't see a real distinction between the evil Canadians eating our bodies or not. In either scenario, they are out to kill all of us and they are not abiding by the rules of war (i.e. - the HCs and the GCs) at all. In that case, then we are truly back to pre-modern rules of warfare as the HCs and GCs explicitly let us off the hook so that we can fight without our hands tied behind our backs.
At that point you get into the pre-modern and medieval ideas of just war, reciprocity, etc. However, none of these ideas are binding in international law today, so far as I am aware.
Another example would be what if the Canadians launched preemptive nuclear strikes at the US with the intent to kill everyone? At that point we certainly have the right to respond in kind to try and survive. The usually accepted rules of war go right out the window.
@The Reality Kid: The laws of war were established to try and make war slightly less horrible. But, they were all established with the understanding that all combatants would be bound with them so that one side would not have an unfair advantage by ignoring the rules. All the rules have escape clauses that allow Parties to basically fight down at the same level as their enemy.