The Geneva Convention on Asphyxiating Poisons and Other Gases

In 1925, the great power signed the Geneva Protocol on asphyxiating poisonous and other gases. The only two major powers who did not ratify the proposal were the United States and Japan. There was an exemption in it that if you were attacked with chemical weapons by a state, you could use chemical weapons in defense of your country. And there was another provision that allowed chemical weapons to be used on any states or groups that had not signed the treaty.