Well should there ever be an end to the US-Japan alliance, survival would be of upmost importance for Tokyo, so nuclear weapon would be the only option. Given the examples of NK and China in the 1960s, no amount of sanctions would stop a determined pursuer of nukes even at the cost of great economic hardship. That leaves a majour all-out war the only option (and even without US involvement, countries like India and Australia would likely join Japan in a last ditched effort to weaken China geopolitically) except China would have no choice but becoming the aggressor this time around. The most troubling scenario for China would be if the US were to decide to terminate its alliance with Japan whilst guaranteeing a nuclear umbrella for Japan up to the day when Japan’s nukes enters LOW status.