That depends. As someone who has an interest in this kind of thing, I can firmly say that if North Korea attack first, even with their nuclear weapons, the regime will get pounded into the dirt and you suddenly have 25 million people who have been brainwashed their entire lives taking up space in China or the Reunified Korea, which spells a humanitarian crisis. China will not assist them because of a deal with America and the South, and America wouldn't attack at risk of pissing off China who still have to pretend they care.
The ultimate question is why would either side attack? Neither will gain much from it, so what could be possibly accomplished?
To answer your questions:
1. China gets involved and it becomes a quagmire of a cluserfuck.
2. North Korea is pounded into the dirt, possible a guerrilla war much like what Russia Experienced following the Chechnian Wars. This is also the likely outcome of 1.
3. North Korea is pounded into the dirt.
The ultimate question is why would either side attack? Neither will gain much from it, so what could be possibly accomplished?
To answer your questions:
1. China gets involved and it becomes a quagmire of a cluserfuck.
2. North Korea is pounded into the dirt, possible a guerrilla war much like what Russia Experienced following the Chechnian Wars. This is also the likely outcome of 1.
3. North Korea is pounded into the dirt.