I think that it will lead to a swift end to North Korea as we know it. Even if the US doesn't nuke NK they would probably take out their leadership within 24 hours of the attack. In addition to that, I think the US missile defence system could stop the missile before it reached the mainland.
I don't think that either China or Russia care enough about NK for them to go to war over it. They even agreed with the 2006 and 2009 UN resolutions.