The NORKS can probably field a localized, small version that could knock out, say So Cal. Sure that would be tragic, and a lot of people would die. But we would recover pretty quickly.
China and Russia probably have units large enough—plus the delivery systems to get them where they need to be. That would be a concern, but in that case MAD still exists and they do not have a death wish.
So, its a serious threat. Serious if it happens over YOU. Not full blown dark ages stuff that fill novels.
It took an entire ARMY to get one (ONE) terrorist in Boston.
Try to imagine answering a thousand miles of devastation. This country is not capable of handling it. Then you wopuld have the Cascade Phenomenon.
Catastrophic Cascade. (Look that one up)