You can't treat any nuclear threat lightly. For the same reasons a bomb threat in a building everyone is forced to evacuate, even when you believe the threat is bogus, there's been one every day, and 90% of the time terrorists explode bombs without warning.
I'll give you an example. Someone is pointing a gun at you, it's a really rusty, old piece of junk. You're not even sure it's loaded, but the man, a crazy one, is aiming at you and threatening to shot. You can risk going carelessly against this man, or you can act cautious that gun may actually fire. It doesn't have to be a Desert Eagle to kill or wound you.
Honestly, a worthwhile response. Yes, you can't treat _every_ threat carelessly. However, the amount of evidence of the military capability of the North is pretty slim. We know that they're starving. Their proof that they detonated a nuke when others were not able to detect the seismograph shock-waves, is pretty non-existent. I can understand if Iran says that they have a bomb, 5 hours after a spike was detected from the region of shockwaves, but this just didn't happen.
To be blunt, I'm not convinced that they have the resources or the technical know-how to do this.
The atomic bomb is actually a very primitive technology. The trigger mechanism is really basic.
What's hard was to develop all the necessary scientific background to understand how it should be made (and whether it was possible and why), and the really hard part is refining Uranium; which they got that covered thanks to the left-over factories courtesy of their USSR bodies.
Little Boy was dropped 50 years without having ever been tested (Trinity was plutonium-based). Technological advances until today have greatly improved their efficiency and power though.
Look, I know that the USSR was pretty loose when it came to AKs and some other weapons systems, but when it came to their nukes -- when the Iron Curtain was up -- those were pretty much untouchable. They might pay lip-service to NK, give them planes, tanks, boats, etc. but no way in hell would they give them nukes.
As for NK getting the stuff on the black market, possible, yes, but again, if they said that they blew a nuke up, then they'd get noticed right away by a number of advanced countries (Russia, US, France, etc.) Things like this are impossible to hide.
There's a lot of ways to detonate a bomb. It is believed NK's technology is far below of what US' can protect. That is, assuming the US' anti-missile systems function as they should. External factors (such as powerful solar storms like the one from today have to be weighted in) can affect their capability. Nothing's fool proof.
I know that, but I'm not arguing that America's missile defense capabilities are fool-proof
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You don't even need to target a city. Targetting something like the North pole, Alaska's block of ice, or Groenland could cause a damage that ranges from meaningless to catastrophic for everybody.