Nuclear armed Korea/Japan is an incredibly huge provocation which I think even their own populations won't support. Japan has a huge anti nuke lobby.
The lobby exists but is limited to protesting, it has no political power. A few thousand anti-nuclear protesters =/= the will of the Japanese people, and unfortunately, anti-Chinese sentiment and fears over North Korea will probably push the public towards acquiring nuclear weapons in some manner. A lot of Japanese don't care that much about politics unfortunately, and those who do are LDP supporters, while at the same time, the progressive elements of the Japanese political scene are weak and lack organization to fight against things like nuclear armament. An example of this can be seen in the 2021 elections in which the opposition completely failed to oust the LDP despite Abe and Suga's failure with the COVID-19 response.
When we look at provocations, we need to understand why they are being made. They aren't blanket imperialism, they are serious attempts to gain or consolidate their control and influence over the world. The West tried to expand into Ukraine by making it a member of the EU and NATO, and it failed when the conflict broke out. Now, the West will be fearful of losing more of its possessions, and will attempt to consolidate what it has. Putin's threat of nuclear weapons use will be taken very seriously, and as a result the West will feel that nuclear weapons are no longer "Cold War relics" and are now usable components of their militaries.
So nuclear weapons deployment will be necessary to both defend their "possessions" of South Korea and Japan, and in order to facilitate hostile behavior towards China. Even if it requires meddling in elections or deliberately provoking conflict elsewhere to influence political forces in the two countries (by either the US or South Korea and Japan themselves), if they want to do it, they will do it; China likely has no hope of stopping them IMO. Doing so would require supporting military coups, which would be difficult to manufacture.
There is a possibility it could be stopped through China's economic ties to the two countries, but based on the European response to Ukraine, I am a little skeptical of how much that will be worth. It is still a possibility though.
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I'll add something to my comments. There is a very, very remote possibility of the downfall of Putin and the return of the USSR. Those are things predicted by the West and they contradict each other, but in my scenario, they do not happen in the fantastical, delusional manner the West thinks they will.
Ever since the dissolution of the Stalin-era internal security apparatus, Soviet, and now Russian, leaders rely on the support of the bureaucracy (often called "oligarchy"), military, and internal security services to remain in power. Most Soviet leaders were good at managing these three entities and remained in power until their deaths. Yeltsin served to his term limit, Medvedev was too liberal and thus replaced by Putin. In Gorbachev's case, the bureaucracy outgrew the Communist Party's political structure, and supported efforts to topple it when the Soviet Union fell (the military and internal security services did not have the organization necessary to prevent this from happening, hence the failed August Coup).
Even if Russia wins, the consequences will have been severe. Putin's decisions seem to have caused tactical problems for the Russian forces; ruining the reputation of the Russian military and equipment, along with causing unnecessary loss of life. The economic consequences will likely be negative as well; I'll get to this below. The FSB does not serve Putin; it serves the interests of Russia and thus is liable to turn against him.
While China can help alleviate the economic consequences, the fact remains that they happened. The bureaucracy may not trust Putin to continue to make responsible decisions for the country; this is exacerbated by his nuclear threats. The bureaucracy took the Cuban Missile Crisis very seriously when they decided to back Brezhnev against Khrushchev.
The bureaucracy would need to wait a couple years after the Ukraine conflict ends to execute their coup. Assuming the conflict ends this year, that should be just in time for the 2024 presidential election. The coup, however, will begin long before that.
It will start with influencing elections in favor of Communist Party of the Russian Federation (CPRF) candidates at the local level and in the Duma. This will erode the current political leadership, which has gone all in on Putin and can't really be salvaged- that is, they are in the same position that Communist Party of the Soviet Union (CPSU) politicians were during the fall of the USSR in 1991. Then comes 2024. Either the proposal to let presidents run for a third term fails in the Duma, or Putin loses the election and CPRF wins, with the election being influenced. Throughout the whole coup, elections would be influenced under the guise of "preventing foreign influence in elections", allowing it to be done with Putin's approval and without his knowledge at the same time.
CPRF wins, and the constitution is soon amended to return the Russian Federation to the Russian Soviet Federative Socialist Republic. Either a similar process takes place in Belarus, or a blatant FSB backed coup occurs to install a communist government. Assuming Ukraine is made neutral or pro-Russian as a result of the conflict, this happens there too. The three then "voluntarily" reunite into the USSR. This time, no right to secession shall be allowed, in order to avert a repeat of 1991.
Ironically, the closest historical example to my theoretical coup is the fall of the USSR and Gorbachev, in which pro-independence candidates got elected into leadership and parliamental positions, and then got rid of the USSR and Gorbachev. In this case, the USSR will be brought back together and Putin will be deposed.
Does this benefit the three key entities of Russia?-
1. For the bureaucracy? Yes: They keep their positions (CPRF praises Dengism and will look very similar to China eventually) and influence, as well as profit, they also have an ideology that both secures their development/profits and expands them.
2. For the military? Yes: the stance against NATO is solidified; there will be no Medvedev style rapprochement. A healthy development of the armed forces can also be ensured, no budget cuts and poor reorganizations.
3. For the security services? Yes: obvious reasons. Lots of work for them to do, their influence will expand significantly, and they will get plenty of funding. No longer hampered by the appearance of a Western style democracy.
4. All three? Yes: CPRF's China style approach will eventually raise the standard of living of the population, ensuring support. There will be fewer issues for the West to exploit in trying to destabilize Russia and bring it in to the Western sphere of influence. The CPRF also promotes cooperation with the Russian Orthodox Church and thus does not pose a threat to any religious interests the three entities hold.
Most importantly, the revival of the Soviet Union allows Belarus and Ukraine to be directly brought under the protection of Russia, completely eliminating the possibility of a second attempt by the West at bringing them into the EU and NATO, while at the same time avoiding the controversial and imperialistic "revival of the Russian Empire", which would create issues with the local population that have a higher chance of causing another collapse-type situation for Russia.
Does this benefit China? Yes!-
1. Russia and China will be ideologically aligned, completely eliminating the possibility of a post-Second Cold War confrontation between Russia and China. The CPRF praises Stalin and China and thus should not pose a threat.
2. CPRF's warm attitude to China will increase already plentiful economic opportunities; not only military, but in the civilian sphere, a reverse of the 1950s when the Soviet Union provided assistance to China might be seen.
3. The revival of the USSR would put immense pressure on the West. The word "Soviet" sends fear down the spine of every Western politician; the West would be forced to expand their conventional military and nuclear arsenals back to Cold War levels, while engaging in dicey economic warfare, all at a time when they are hurting the most. This will not only pressure Europe, but also Japan and South Korea.
4. Upon the completion of the new USSR's modernization, China will have an ally equal to itself to face off against the West. The global confrontation will basically be a reverse of the Cold War, with a healthy China and USSR competing against the dilapidated West. Their policy of non-interventionism will encourage support among other countries, and their successful development models, especially Russia's modernization, could even pave the way for the establishment of similar governments in other countries by pure popular support, eliminating the possibility of the rise of revanchist governments following a hypothetical demise of the US and its Western allies.
Most importantly, there will be absolutely zero threat of a reverse Nixon. After years of demonization, the West is simply too emotionally and politically immature to deal with "the Soviet Union" in a friendly manner, especially not one that openly praises Stalin.
There is a 0.00000000000000001% chance of this happening, but I thought examining the domestic Russian situation in a more extreme scenario was worth doing.