In WW1, we used phosgene and mustard gas. During WW2, we had chemical agents deployed all over Europe and parts of Asia to be used against Germany and Japan but it was [thankfully] never deemed necessary. Use of agent orange (allegedly) and other chemical agents (not allegedly) in Vietnam. Other allegations (unproven) of use in various Cold War conflicts, namely Korea. Many, many, many thousands of American soldiers over the past 100ish years subjected to chemical weapons testing (mostly minorities).
We still had missiles armed with VX gas and other chemical/nerve agents up until like 2022.
All it would take is one psychotic administration with a Nazi running the Pentagon to bring shit like this back. Pretending it's all part of the past and we've moved on from shit like this is exactly the way you end up repeating history.