How the China-North Korea equation is changing

ALL serious governments think, hard, about unthinkable horrors. For America, China and other Asia-Pacific powers, few potential events are as grim to contemplate as a war involving North Korea, or that country’s violent collapse.

There are reasons why the world does not seek to topple North Korea’s impetuous young leader, Kim Jong Un. For one, his regime—a Stalinist take on a feudal monarchy, funded by mafia-like criminality around the globe—keeps artillery pieces and rocket-launchers aimed at the South’s booming capital, Seoul, 35 miles from the border between the two Koreas. To convey the costs of that conflict, American experts recall the grimmest examples of urban destruction in Chechnya, and imagine evacuating millions of civilians from Seoul and its suburbs, under fire.

Chinese leaders have their own nightmare scenario: the chaotic fall of the Kim regime, sending millions of refugees into north-eastern China as a race begins for control of the North’s nuclear...