Like OP said, I agree it is worrying, they have done this before not only the threats but actual action being taken.  US don't have the worries that others will and do have.

There is a bit of a pattern here and again like most little bullies they will no doubt pick on the weaker nearer link  like S Korea...this did happen in 2010....

I wonder who if anyone is pulling this little upstarts strings...I'm not sure why but I just get that impression.

Also it is a worry when others aid, and jump on the bandwagon, for their own reasons, people like this who offer help or encourage them to 'kick off' so to speak.

Our planet needs help, not more destruction. 

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I am of this world, but not always happy to be a part of it...