Their motivations were political and not religious, (yeah I know some said communism was a religion for them). They also knew that any such attack would be blamed on them, at least and probably outright traced to them. Then it was odds on to start a nuclear war.

As volatile as our relationship was, I think they wanted to win fair and square, by burying us as we wound up burying them. They damn sure did not want a nuclear exchange with us after their agony in WWII.

My fear has been the loss of control over fissionable material after the fall of the USSR. There are any number of Muslim terrorist groups that hate us enough to nuke a city or two.

I suspect that the only thing that has stopped them is the fear that the US under martial law might declare total war and start nuking the holy places. They invented death by a thousand cuts and they probably prefer it.

Domestic terrorists are probably quite willing to do it but they might not have the connections needed yet.