North Korea won't start an all-out war with USA. If they did, they would lose all sympathy from both Russia and China. Even the dumbest person would have enough understanding of consequences to not do something so foolhardy.
So that's it, right? North Korea is just baring its teeth and the human race isn't stupid enough to actually start a nuclear war, right?
I'm afraid it's not that simple. Here's how i believe the lead-up to a nuclear war might unfold.
North Korea keeps working on their missile launch technology, which they of course will.
Meanwhile, things escalate somewhere in the Middle-East, for example in Syria or between Israel and Iran.
Syria has very closely ties to Russia, and the US has at least since 2013 considered overthrowing their current regime. Even though the situation is mostly on hold, both forces are currently arming up on each side and there are minor clashes between the two forces. If this keeps up, it will eventually escalate and bigger clashes will gradually happen.
Israel and Iran are two nations that have a lot of hostility between each other, some of it because of nuclear arms, some because of disagreements about national legitimacy, but also other things. What makes this situation so tense is that Israel is allied with the USA, while Iran is allied with Russia.
Recently the two have had negotiations about their nuclear deal, which seems to cause even more hostility between them.
If Israel and Iran can't learn to work together, eventually one of them will turn to their ally and say "These people won't work with us, they are even threatening us", thus the ally will send them troops and weapons. The other side will then tell their ally "They are arming up for war, and we need your help", so both sides will arm up.
These two nations do not border one another, so a direct war might not happen at first, but it's not unlikely that one of the two forces will bring Iraq into this by stationing soldiers there, either by invasion, or by an agreement.
It does not matter if things escalate in Syria, or between Israel and Iran, the two forces will eventually feel justified in attacking other allies of their opposite, either to harm their resource supplies for their soldiers, to gain strategic bases, new political allies, or something else entirely. This will potentially bring neighboring nations into the hot mess, and before we know it, just like with WWI, the chaos will reach a global scale.
Exactly how things will unfold from here is hard to predict, but the division between the two sides will be a lot bigger, and a direct war between them on many fronts is probably going to happen.
Since China already has stationed troops in the Middle-East to aid Russia, they will also be pulled into this fighting.
Now, as the biggest forces of the world are fighting, their smaller allies will be able to start wars with their own neighbors if they are allied with the opposite side, and they will still be protected.
This of course is where we come back to North Korea, a nation that never officially ended their war with South Korea, and has regularly showed how militant it is. They will most likely take this chance to invade South Korea. USA will of course react by sending in troops themselves to protect the South.
As soon as US troops set their feet in the North, or maybe just if North Korea faces a major loss, they will have the excuse they've been looking for to send a nuclear strike to the USA.
USA will of course respond with their own nuke, but since the division between the two forces of the world has already gotten so large, and North Korea technically didn't start the war against USA, Russia and China will still support North Korea in their attack, and try to defend them from a nuclear missile.
Whether North Korea is defended or not is not relevant, what matters is that both sides have now used nukes, and they both feel justified in doing so again, so there is no telling how great the destructions will be.
This is, of course not something i want, i don't think any rational person would want this to happen, but i believe it is a possibility, if even just a slight one.
We should also remember that events similar to these have been seen before, and a nuclear war was still avoided. And it can be avoided again.
