Make no mistake, this situation has the potential to have catastrophic consequences.
We simply do not know the extent to which China has influence over North Korea. Tension between China and Japan is increasing. The US sees Japan as being pivotal to its influence in the Pacific.
South Korea, Japan and China have much to lose economically by armed conflict. North Korea has nothing to lose. Totalitarian dictators are not marked out for their logic, see Saddam's invasion of Kuwait.
A strike against US forces is a possibility, and one which the US could easily deal with and respond to. A land battle within the Korean peninsular would be much more difficult to respond to.