North Korea borders both and they won't want a unified US backed Korea with American military bases on their land border.
No they wont, but they wont go to war over it, they have already got US bases on their borders. Infact in some respects they may prefer that than having some very unstable people in charge of an increasingly large nuclear arsenal. After all Moscow and Beijing will be within range of Kims missiles.
I think that you are both right and I think that there may be a modern version of the Cuban missile crisis looming.
I can see China and Russia being 'eventually persuaded' to force NK to disarm, because they become convinced that Trump will undertake a pre-emptive strike.
I am definitely no fan of Trumps but the UN has been an impotent force for decades - they will not be used to such blunt speaking - but a lot of people have been thinking for decades that the UN needs to find/re-establish its purpose and having it called out as such may not be a bad thing?
USA warships can easily prevent (I am guessing to a degree) any missiles launched by NK from going very far.
I think that he is serious - and on this subject I am not convinced it is a bad thing - NK needs to be (nuclear) disarmed