Look I don’t want to be repetitive since this talk was exhausted, but the US secret services are basically omniscient; they know everything out there. If Russia’s WMD capabilities were as doubtful as you say, no doubt the US would know. And at that point, what’d hold them back from not only wiping out the Russians in Ukraine, but also invading Moscow? I mean Russia is the US’ mortal enemy, after all.
The real reason is there is indeed some degree of serious threat from Russia they hold the cards to that prevents the West from doing that.