It comes in 2 forms:
1. An agreement by both sides. Both belligerents agree, Gas is bad. So neither side uses gas. If one side disagrees, well that’s that, but if you disagree, that means the enemy can use it too, and that’s why the bans are followed. Neither side wants to be gassed, e.g.
Breaking treaties is also grounds for war anyway.
2. Victor’s Justice. Beating the other guys in war allows you to enforce bans even if the other side disagrees.
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