I've always regarded mainstream news with suspicion, so I never take anything too seriously and don't know what to believe and trust, so I'm not too familiar with the whole North Korea situation. But I do know of all the talks about North Korea having nukes and testing them and all the threats by Un etc.

How can this puny country be threatening the world when the rest of the world can just gang up on it and take it out? That's what I don't understand. As I said I haven't been following along too closely, but aren't they supposed to be somewhat isolated in terms of alliances with other countries? And where do they get money, technology etc. if, again, they're supposed to be isolated from most of the world?

What would happen if the US just decided to declare war on Un and take him out? Would any other country support Un?

North Korea does not want to be owned by the nєω ωσrℓ∂ σr∂єr.
They have their own currency.  They want to remain a sovereign nation.
Like Iran, Syria, Bolivia and a few others.  

North Korea was treated very badly by the US in the past, so they need
nukes to protect themselves from the US attempt to destroy all sovereign
nations (only a few left).

Of course the big bully of the world does not like for them to have nukes,
just like the big bully does not like for Americans to have guns to protect
themselves from the big bully.

See http://paulcraigroberts.org for more details.

Oh so North Korea is actually the good guy here? 

The global elites have been planning WW3 since the day WW2 ended.  WW3 is part of their "end game" - to kill 1/2 the population, to usher in global government and the one-world religion under the anti-christ.  Go look up Albert Pike's plan for the 3 world wars and what he wanted out of each.
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North Korea has been controlled by the deep state/c1a for decades.  It's been used as an excuse for the US to spend trillions of $ on "military spending" (which much of that was stolen) and also as an excuse to coerce other nations to join anti-nuke treaties in an effort for world peace.  Basically, north korea is used an excuse so that the US can have a military presence in asia/europe, which gives them an opportunity to "poke the bear" of russia, who is the planned enemy for WW3.  
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Trump liberated N Korea from the deep state and is trying to neutralize this threat.  Trump is also trying to be friends with Putin, even though the deep state is trying to start WW3 through multiple means (i.e. they continue to "poke the bear"):  1) skirmishes in Syria, which threaten russian oil pipelines.  2) skirmishes in Iran, which threaten one of russia's top allies.  3) the multi-decade buildup of NATO, which has expanded from a simple defense of europe into a surrounding/threatening army at much of russia's border.  4) the deep state overthrow of ukraine's govt a few years back, from a pro-russia govt into a pro-west goverment was the main reason that russia "invaded" ukraine (the country was in chaos after the West got involved, and russia liberated their people, who wanted to be allied with russia, not the US).
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There's lots more to all of this.  This is why the media hates that putin/trump are working together.  We can only hope they are trying to avoid WW3.

It’s just a plan dude.  Plans can change and be foiled.  God allows evil to triumph and be defeated according to His will.  Korea is a small part of the puzzle/plan.

As an example, birth control is now used almost everywhere in the west.  Yet it was invented in the 40s, almost 80 yrs ago.  Took that long to condition the people and to erode morality so that the population in the West would decrease.  But that part of plan has succeeded.

You can look at modern history in 2 ways - accidental or planned.  Most major events were planned.