It's a war of nerves. It isn't only the North Koreans who may be building bombs, and the Japanese would be much, much better at it in any case - and unlike their NK counterparts, they do have plenty of material. China is waiting for us to make the first move and we are waiting for them. If nobody moves, the world changes. If we move together, the world changes. It will be a fine calculation on the part of the Chinese which world will be more advantageous to them. My guess is that one with both North Korea and Japan armed with nuclear weapons probably isn't the best option, if indeed they have any real option left at all. Loss of the option changes the world as well. "May you live in interesting times!"
Are the Japanese building a bomb?