That's a load of crap as well. A chem agent is a bio agent is a nuke. Guess what? We only have one of those and we will use them.
Plans during the 70's called for first tactical nuclear strike on the Soviet armored forces if they broke through the Fulda Gap in a Germany invasion and it looked like we were about to lose.
We used nukes first on Japan and we have NEVER ruled out first use.
it won't happen. decades of political correctness has hollowed out that kind of determination within our government - and most of our citizens.