It's always the same game since WW2. USA and [insert superpower of choice here] have issues with each other but can't go to war because nobody can win a full out war anymore. The look for some poor countries to act as their puppets and start a war there. That's the way things worked in the cold war vs the SU, and that's more or less the same way the USA seems to think now.
Problem: this does not work anymore. All this "north korea can't beat us" salmon here...it's worth nothing. Just imagine...if the US would really start to attack north korea. What could happen:
1) NK uses their nukes, either on south korea or on the US military present there (I won't even go so far as smuggling a small nuke to NY...)
2) The NK are as bad martial wise as people here *claim* they are: the US slaughters an army of 2 millions and then occupies NK. They will loose most of the little respect they have internationally...and posses a country as problematic as the iraq- without the benefit of having oil.
3) They get dragged into a long war, which with each passing year gets harder and harder to explain.
IMHO, there is no way the US can "win" in this scenario. And NK's acting shows they know this.
They rattle their sabres, get attention (and maybe some under the hand deals) and that's it.
Sadly, NK's example shows a basic way which will be followed by many states: if you have nukes, your safe from US hegemony (more or less).

Cheers
FF
There are 10 kind of people in the world.
Those who understand binary numeral system, and those who don't.