China was ask US and EU to coordinate global sanctions and render Japan to be a second North Korea economic basketcase.If there is any indication that Japan is building nukes, what does China do then? At some point the CPC will be faced with the prospect of biting the bullet and doing preemptive action.
If US or EU refuses to participate in sanctions, then China will threaten to arm every single neighbor or enemy of US/EU, such as Venezuela, Iran, Mexico, Cuba, etc. ..with nuclear-tipped ICBMs, as proliferation is a two-way street, it doesn't mean only China's enemies get nukes. Everyone of US enemies will get nukes.
Just the prospect of "mutually assured proliferation" (MAP) will trigger US/US to participate in global sanctions regime against Japan, and Japan cannot survive economically with such immense economic pressure, they will fold.