China has already stated that if the United Stats *starts* a conflict with North Korea, they will intervene on the side of the Koreans. If North Korea starts it, they'll just sit and watch. Russia has made no statement either way, but I think that Putin would want to stay aloof and look to the world like *he* is the reasonable one, not Trump (which may or may not be true....).
I do think that it could rapidly escalate into a theater-wide nuclear war (as in, only nukes on the Korean peninsula). North Korea has more than a million standing soldiers with roughly another million in reserve. If we went to war with them, we would he horrifically outnumbered, and even our vast technological edge wouldn't likely be able to stop an invasion. In which case, Trump might decide that popping a nuclear weapon could kill enough people, or destroy the right place, to end the war (or maybe he just wants an excuse to use them). North Korea would respond with nuclear weapons, if they actually have a means of delivering it. I doubt that they would be successful getting a nuclear weapon over South Korea, but it would still be responding to a nuclear attack.
However. I don't think that Kim Jong-Un and Donald J. Trump have enough brains between them to actually know how to hold a war, and share the secret terror that makes them wet themselves when someone says, "Boo." I sleep just fine, not expecting a nuclear *or* conventional war in Korea.