Plough,
From various statements out of the USA I reckon they would reply militarily immediately. I think even Malcolm Turnbull has confirmed that we would back the US in such a case which seems to confirm the military reply option.
I caught the end of a recent interview with Professor Joe Siracusa (RMIT) on TV (not sure which channel) where he said that even if NK didn't attack, the US might still carry out a targeted strike on North Korea's ICBM missile silos which prevents NK from reaching Japan or Guam etc, which only leaves South Korea as a target. The gamble is that the NK regime is a lot more sensible than we give them credit for so they wouldn't attack South Korea for fear of risking millions of lives, including their own. They would come to the negotiating table instead. Part of his rationale was that the NK missile silos would take a couple of hours to prepare for launch and they would be hit well before that so they could only attack South Korea using conventional weapons. That's a huge gamble, IMO.
