@Lash,
The US, although it would be a costly victory.
I don't see anything happening because China is putting pressure on them now, but additional threats from NK were inevitable.
Control over a "unified" Korea has long been the Holy Grail of the regime.
The world has made it absolutely clear that it doesn't care one whit for the suffering of the NK people under this regime and no significant effort has been made to change the regime.
So why does the regime want more nukes? So when they drive another million North Koreans to starve to death they can be sure the world won't try and forcefully come to their rescue?
The only action they might take that will prompt intervention from the rest of the world and result in the elimination of the regime is an all out invasion of South Korea. If they had a means to send nuclear missiles against the intervening nations' homelands it could be the deterrent they seek. The rest of the world might shrug it's shoulders and say "It's up to the South Korean army."
Let's assume NK wins a war with SK. What then? The Fat Little Tyrant becomes content with putting his pudgy foot on the necks of all Koreans or, flushed with victory, does he entertain ideas of "paying back" the Japanese for WWII? Who knows but it's rarely safe to bet on insane tyrants ever feeling satisfied with whatever level of power they have.
If you believe a face-off is inevitable (and I do) then it's better to get it over with sooner than later. It should have been done decades ago, but strategists assumed that the Kim dynasty could not continue. Seems like the third monster is worse than the prior two.