My feeble contribution:
1. NK is a vassal state of Russia, not China (contrary to widespread thought), given that the KGB owned the Kim dynasty from the beginning.
2. NK's recent bellicose statements have led to the USA parking more anti-ballistic missile "shielding" closer to Russia far eastern periphery, thus enabling Putin to go on TV in Russia - as his puppet Medvedev did in recent years - saying, "See, didn't we tell you the USA is encircling Mother Russia?"
3. Russia's ICBM forces are highly modernised (contrary again to common beliefs) and are numerically double the USA's and with larger yields per warhead, thus meaning Russia is geared up for a first strike - if anyone is going to go starting WW3, since we're talking about it.
4. The USA places greater emphasis on a retaliatory deterrent - hence its large nuclear submarine fleet. Its policy is to absorb a first strike since 1997 when Clinton introduced PDD-60, to take an ICBM saturation strike, propagandise the surviving masses, then take Russia on in a more conventional war.
5. MAD has not existed since the Cold War because "Launch on Warning" is no longer official US policy - see # 4 above and besides, modern yields are much lower than was the case in the Cold War because delivery systems are much more accurate, thus obviating the need for the sledgehammer, environment-ruining approach.
6. Nuclear winter is an unproven concept and is radically reduced in impact anyway, for the reason outlined in #5 above.
7. AFAIK, the USA has always modelled nuclear war in Korean Peninsula on the the basis of a land invasion by NK against SK, leading to the USA using very low yield tactical nuclear weapons - such as those carried by the B-52 and B-2 bombers that flew over SK this month (thus provoking NK's recent threats).
8. NK has a long history of threatening SK and the USA, but the more recent outbursts may be interpreted as a kind of probing exercise, to get a reaction out of the USA. Why? Because Russia has argued that the USA is continually probing it with threats of missile shields in eastern Europe and the Far East. Here's a quote for ya: “I would add something about what we have had to face in recent years: what is it? It is the construction of a global missile defence system, the installation of military bases around Russia, the unbridled expansion of NATO and other similar ‘presents’ for Russia we therefore have every reason to believe that they are simply testing our strength.” - Dmitry Medvedev, Russian President (at the time), 23rd November 2011.
So... that's my thoughts on it FWIW. NK is Russia's client serving longer term Russian geostrategic aims, i.e., the vilification of the USA in advance of economic warfare (e.g. Russia is trying to set up a new monetary system outside of US influence) and dear knows, maybe the real thing sooner rather than later.