I think the Guam announcement could just be a diversion, real attack could come from a smuggled nuke in a shipping container to a west port city. His ego suggests he wants to strike the mainland.
The smuggled nuc has always been my worry. One bomb, just one, on Manhattan at 23d street or in the Reflecting Pool area in DC and America stops.
Such a bomb would kill no more than 100,000 at most, but the resulting food shortages, etc, businesses stopping, etc, would kill tens of millions.