However, it must be remembered that China has STRATEGIC AND TACTICAL NUCLEAR WEAPONS, WHILE JAPAN HAS NEITHER.
ADVANTAGE = CHINA.
Also, given the high risk of US intervention on the side of Japan vee the US-Japan Security Treaty, IT IS HIGHLY LIKELY THAT CHINA WILL UNILATERALLY OR PREEMPTIVELY ATTACK JAPAN FIRST IN MASSIVE CONVENTIONAL OR LIMITED NUCLEAR [Tactical Nukes] BARRAGES IN ORDER TO FORCE JAPAN TO SURRENDER OR GIVE UP BEFORE US MIL FORCES ARRIVE IN-THEATER.
ADVANTAGE = CHINA, again.
Perhaps more importantly, CHINA HAS A HISTORY OF CHALLENGING + FIGHTING MAJOR POWERS WHOSE NUCLEAR + MILITARY TECHS IS WORLD-ACKNOWLEDGED AS "SUPERIOR" TO THAT OF CHINA. China fought agz these anyway.
IOW, China is more likely to fight than NOT fight,+ its NOT going to matter to China that Japan's ally is the Nuclear-armed + "Sole" Superpower USA.
What China ultimately wants is TAIWAN - it desires the Japanese Senkakus [China = Diaoyus]+ Okinawa? to guard the maritime approaches to, from WESTPAC vee its desired future PLA MilBases on [post-Unification] Taiwan. WIDOUT CONTROL OF TAIWAN, IN CHINA'S MIND THERE IS NO "POST-US", "WORLD #1", "MANIFEST DESTINY" FOR CHINA, AT LEAST NOT IN THE 21ST CENTURY.
China will have to wait GRUDGINGLY for many more decades, iff not sometime in the 22nd century, to see its "Manifest Destiny" achieved, iff at all. RATHER THAN WAIT FOR AN UNCERTAIN FUTURE, CHINA IS PRAGMATICALLY TAKING ACTION NOW.