If nothing else, President Trump’s Korea policy has been a master class in trolling. For months, talk of “maximum pressure” and “all options are on the table” had foreign-policy experts on fainting couches over the prospect of imminent nuclear war. Then Mr. Trump announced the summit with Kim Jong Un and the establishment turned on a dime, solemnly warning that a mindlessly dovish president would give away the store.

Then Mr. Trump canceled the summit. This was an amateur move, critics sniffed; the flailing of a dilettante...