Well, it holds USA&Co for now, but what if NK fires first?
Will a NK nuclear strike really cause a nuclear counter-strike?
Or, will those who lead our countries have enough sense to deal with it in another way?
Don't forget the automatisms of the world. If the US soil is attacked, the NATO will have to respond with the USA. This will not be a small skirmish then, like the Kosovo conflict, but the full machinery will start to run before any US politician could shout stop. The other NATO members will automatically turn into a pre-war state, assuming it is handled everywhere else similar to Germany: In such a situation, we would have the "V-Fall" ("Verteidigungsfall"/defense case), and the military would react instinctively. Reservists will be activated, mothballed equipment will be readied for activation, all bases will be in alert. If needed, the whole country could be ready to start fighting with the rest of the NATO in hours, and reach full mobilization in about 10 days.
There is no automatism saying, that nuclear strikes have to be responded nuclear. But the autopilot of the world would at least mean that the USA get ready to respond, and with them, the rest of the world's nuclear nations.
But considering that NK has only enough material for maximal 12 nuclear fission warheads (which is peanuts), a nuclear response is not automatically necessary. Harder would be estimating the strength of the NK military. Officially, NK has 1.2 million soldiers - armed to the teeth is a good description for the country. If they would be all in good health, moral and equipment, a serious opponent, even for the USA. Also the air defense network of NK is pretty dense, the air supremacy of the USA would be at least limited.
Depending on how much of it is true, if the NATO decides to answer with war, a series of nuclear strikes against key positions of the NK military would be thinkable, to move the odds further towards the NATO. Nobody would like to see hundred thousands of dead NATO soldiers. But if the political situation with the other countries (especially China) would be too hot to handle, it is likely that the nuclear option will not be taken, and a full blown war starting out from Japan and Korea would be the case.