Chapter 20 studies gas, biological, chemical, and nuclear weapons, looking at each of those prohibited weapons in turn. Poisonous gases were first banned in 1925 – but only their use, not their production or sale. There have been numerous uses of poisonous gases, including today in Syria. Biological and toxin weapons were banned in 1972, although their use is unmentioned in the 1972 Convention. A 1993 Chemical Weapons Convention, with a strict verification process, has been more effective. Russia’s continued poisoning of state critics is detailed as an example of state-avoidance of the 1993 Convention. CS (“tear gas”) use is limited by the 1993 convention but still employed as a riot control measure. There is no international law that makes nuclear weapons unlawful. Of the several multinational treaties that bear on nuclear weapons, the most significant is the 2017 Treaty on the Prohibition of Nuclear Weapons, which, so far, has but forty-four State Parties. None of the nine nuclear powers have joined, of course. In short, gas, biological, chemical, and nuclear weapons are abhorred and condemned until they are used; then the international community looks the other way.