My theory is that China simply want nothing to do with a conflict in any way and that is the reason they are allied with NK. It's simple but clever of China: If Trump aggresses China is under no obligation to assist the US military action and if NK tries to retaliate or heaven forbid, use Nukes, China can simply declare " We cannot condone or assist such acts". In short, they can sit back and stay well out of it, which is pretty much what they have been doing.
It's just a theory.
It's just a theory.