Technically, the United Nations is already at war with North Korea, and has been since 1950. There was an armistice- meaning a cessation of hostility- in 1953- but there has never been a formal treaty to conclude the war.
In 2009, Kim Jong Il threatened to undo the armistice, but only engaged in minor provocations. (Okay, sinking one ship wasn't so minor, but it wasn't an all out strike either)
If hostilities resume, technically it is the UN - not the US that intervenes. In practice, this means a "coalition of the willing" would need to occur. The "obligation" to send troops is primarily based on commitments to the UN.
The US and South Korea have an Armed Forces Agreement, and forward deployed men. As such, they are pretty likely. Japan is constrained by treaty not to deploy forces outside of Japan. Parts of Europe may fulfill their UN commitments, but Russia would be unlikely to "intervene in the internal affairs" of another country.
The real wild card, as you say, is China. Even in the 1950s, China disagreed with the idea that the UN should declare war, and came to the help of their ally. That they agreed to a censure of their ally truly was a new thing and a shocking development (& most likely exceedingly worrying one for NK).
Even if China simply sits on the sidelines, the outcome of any action would be clear. Remember that the Korean Conflict in the 1950s was a massive invasion by the North which was quickly repulsed once the UN intervened. It was not until China entered that the war became anything other than a lopsided affair.
