Japan has been doubling down on it's rhetoric against China and the new PM has even started a campaign to get rid of the article 9 provisions in the constitution. This has inflamed tensions and China would have a reasonable case for war if Japan landed troops onto sovereign Chinese soil.
But...China wouldn't nuke them?
I mean I'm not saying they wouldn't fight back, obviously, but China has a no first strike policy. Unless Japan launches nuclear weapons first, China wouldn't launch them in the first place. This has been the policy ever since China first developed nuclear weapons. [I mean, the only other use case is that if China actually feels like they need to use it to prevent occupation/invasion of China. That's what the nuclear arsenal has been developed for, to make occupation too costly for imperialist powers. I guess technically you could argue a deployment onto Taiwan would count, but...its kinda just dumb? It's not like Russia nuked Ukraine after it invaded Kursk. Xi isn't sitting at his computer like "hehe yes I love nukes let's blow up people."]
Imo the most likely case would depend on escalation. I think if it's limited to just a conflict over Taiwan island China would probably just blockade Japan and force a deal. [I mean...they could try a ground invasion bit I don't think most Chinese people would actually want to do that given the immense resources required].
Sorry, I just keep seeing it said that "Japan will learn when they realize china has nukes this time," even by non-chinese communists. And honestly it's just...not funny? Like if it is a joke, I don't see the punchline. If it's not, then it's foolish mental behavior at best and outright callous at worse. I'd expect those kinda things from Han nationalists with the username "Yonglesgreatestsoldier" or something.
A little bit off topic, but why TF does right wing nationalism (which is supposed to be all about exclusive self determination of one's own country) always end up compromising with American interests?
We saw this with Ukraine, Britain, Canada, and now Japan. I'm sure there are many many others.
To answer your question, the invasion of China won't be conducted by Japan. It will be a joint task force consisting of the US, South Korea, the Philippians, and Japan.
As it is, a simulation of a Chinese invasion of Taiwan has the US + Allies able to hold off the invasion, but with massive losses, including 2 of its 10 aircraft carriers, Taiwan losing its navy and Japan losing half its fleet of fighter jets and a quarter of its navy. China would lose 1/15 of its air force and 1/6 of its navy.
In short, China doesn't need nukes.
https://socialbites.ca/politics/reimagined-csis-war-gaming-on-taiwan-2026-and-beyond
And it's not just Japan. South Korea and the Philippians are experiencing a military buildup as well. Otherwise these countries do not have the qualifications to invade China alongside the US.
https://www.lowyinstitute.org/the-interpreter/south-korea-track-become-defence-powerhouse
https://www.rappler.com/philippines/defense-budget-2026/
Two things:
One, right-wing nationalism is always anti-communist so they naturally will ally with the dominant Anti-communist power.
Two, a lot of those movements (or at least their most prominent representatives) are often astroturfed by Americans in the first place.