The first flash will be the NK's destruction of Soul. If they do it with their nukes, the Robust Earth Penetrator (gad, how I love that name) will be part of the answer. Otherwise, non-nuke cruise missiles and lots of em.
Only thing I see preventing a nuclear retaliation is China....they are not going to be big on a nuke going of on their continent.
BTW...I love that name too...lol