In a new section of his book on Trump's presidency, The New York Times' Michael Schmidt details John Kelly’s tenure as White House chief of staff from July 2017 to January 2019.
US planned this in Ukraine too. detonate a dirty nuke somewhere in Ukraine and say Russia did it in order to justify more force. Russian media called it out
So who exactly would he have blamed it on? Just about everyone else with nukes either have no motive for dropping a bomb in pyongyang, or allies of the DPRK and wouldn't be believable, or a member of NATO, dragging the US into a war anyway if Korea retaliated.