The world supplies of poison gas and similar agents would surprise even the most knowledgeable among you. Why they have not been used extensively is only because everyone has them and retaliation is too punishing. Some are subtle while others are outrageous. It is entirely possible to take down an entire city the size of New York with a small supply of chemicals. enough to make everyone sick to the point of incapacitation or death. By comparison, Nukes are benign.
And how many chemical weapons do you think the US has today?