The old doomsday clock was about the USSR and US having arsenals of thousands of nuclear missiles to render the world inhospitable if they ever declared open wa

The old doomsday clock was about the USSR and US having arsenals of thousands of nuclear missiles to render the world inhospitable if they ever declared open war. North Korea could probably kill a sizable chunk of the 25 million people living in the metro area of Seoul, but would otherwise not be able to inflict harm on the US, at least not yet. And we would be their nuclear retaliator if they attacked anyone - we know all their military bases, and a nuclear attack on South Korea would see instant retaliation upon all their installations. While China does not want to deal with the refugee crisis that would result from a post-NK scenario, they would definitely never fight the US directly because they know full well that doomsday scenario would play out in nuclear war with the US.