"You're assuming Japan does not already have nuclear weapons."
They don't even need one. All they need is to have all the ingredients in place ready for a secret crash program -- everything short of having to declare they've gone nuclear. They have the wealth, the critical ingredients and the know how...and now the strongest motivation they could possibly have. China has good reason to worry about Japan going nuclear.
You have to admit that it's surprising China has shown such restraint re:Japan.
I've read accounts of what the Japanese did there that made Stalin or the Nazis look like amateurs.