@Builder,
The most likely scenario is that a rogue state such as North Korea, or a rogue organization such as ISIS would make a thermonuclear attack. If common sense prevailed, not always a reasonable assumption when governments acr or react, we would not nuke either North Korea or Syria. The prevailing winds of the northern hemisphere would assure that the fallout from nuking North Korea would soon arrive in North America. We have too many important allies in the middle east to nuke any part of Syria. The appropriate response to a nuclear attack from the Loons in North Korea would be a conventional attack to take out Pyongyang, and whatever of their facilities which have been identified as the source of their nuclear programs--along with Kim's bunker if it has been located. The appropriate response to an Islamist attack would be an all-out ground attack. So far, the air attacks in Syria and Iraq have hurt ISIS pretty badly, despite the BS which they feed the press, and which the press idiotically reports as though it were plausible. That would be costly in American casualties, but would be rewarded with a high degree of success in crushing their operations.
As Sting observed in his 1985 song, the Russians love their children, too. It is unlikely that Russia would initiate a thermonuclear exchange, especially given that theirs is a kleptocracy, and they understand that they could not enjoy their ill-gotten wealth in the wake of such a war.
A full thermonuclear exchange would have a very high probability of wiping out most life on earth. The cockroaches might survive. I wonder what the odds are of a sentient, technological species evolving from irradiated cockroaches.
