Right now North Korea is fueling a missile. They say to launch a satellite. But seeing as they already admit to having Nukes (they say a lot we're not so sure) and there are Iranian Scientists on the ground. . . It really begs the question of how stupid do they think we are. Even if it is a perfectly legitimate satellite, which is extremely doubtful, that missile presents a threat not only to the US but to the entirety of East Asia.
Nukes are a problem that we had hoped had been put to bed with the end of the cold war. We were all friends, sort of, and we thought the old belligerents that had been kept in line by the superpowers seemed to accept that. But as recently as the Clinton administration there were cracks in the post cold war consensus. First North Korea was playing around with Nukes, then the Middle East started acting up. It really isn't worth mentioning Africa, which has never been stable since the old colonial powers ended their rule there.
But for the most part despite the fact that China seemed more belligerent than they had before, they were still notionally friendly. It really wasn't worth mentioning Iran, but they weren't really a threat to anyone. So what happened. Well partially 9/11, which showed that the US could be attacked without a massive military. But more than that the fact that some of the biggest monuments to both capitalism and our military power were nearly destroyed showed the world that it could be destroyed.
Further than that with our whole attention turned to Afghanistan and then Iraq, a lot of the regional powers felt free to act up. Indeed at the height of the Surge in Iraq in 2007 Iran had sent actual troops into Iraq and those special forces really did kill US Service members. What was our response? I wish I could tell you there was one. Now Iran has Nuclear reactors, and the methodology to refine Uranium or Plutonium to make weapons grade fissile materials. What does that mean? Well essentially they have the raw materials to create a nuke, which is actually pretty simple. Worse after the "Axis of Evil" Speech which at the time wasn't technically true Iran saw the benefit of joining with N. Korea. What's scary is that Iran is directly opposed to Saudi Arabia, so any actual Nukes produced will spin off an arms race in the Middle East.
North Korea has pretty much said Japan, South Korea, and most of East Asia is theirs. It seems a tall order seeing as their people are literally starving. Their military seems inordinately strong given that there are people literally starving to death. There are over a million artillery tubes of various sizes alone, that not even speaking of tanks, or infantry. Their Air Force is also pretty large. So taking them on is daunting no matter who you are, and then there's China. They are close allies with N. Korea. So it is possible anything that happens in N. Korea it will involve China.
So what do we do. Well simply put, shoot it down, if we are able. Do not allow any ballistic missile leave N. Korean Airspace. Make it clear that this will not be tolerated. You can not deal in good faith and trust in the honor of a state that has proven time and again it has neither. The same goes with Iran. Anything having to do with Nukes that is not directly related to their civilian power plants should be destroyed. One warning should be given, then one large, single wave should be used to take out the said facilities. When the dust settles we can say whatever we want, but both actions would be both justified and "legal".
The UN will do little more than send strongly worded letters, China and Russia will not intervene, China because they're trying to be a behind the scenes Superpower, to pit these regional bad boys against America. Russia really can't deploy military forces outside its own boarders, despite still having some bad ass forces, the Kursk disaster has pretty much shown the threat of the Old Bear to be a pretty empty one. It will fall to America and our Regional allies. What form or shape those actions will be are debatable but it is clear that no one is coming to the rescue, other than the good ol (and tired) US of A.