In October 1942, the Soviets redeployed their Far Eastern troops to the west, to assist in the fight against the Nazis, based on the intelligence that the threat of Japanese invasion was negligible. In OTL this gamble paid off, but say it didn't, and the Japanese do chose to invade Far Eastern Russia.
The German invaders are repelled pretty much as in OTL, despite the gradual dripping back of troops to the East. As soon as the Germans are pushed out of Soviet territory, the Kremlin decides to concentrate on the Japanese. The writing on the wall is that the plan to push Japan off continental Asia entirely, and perhaps even launch an invasion of the Home Islands. The mood in the West is broadly in favour of this, the Manhattan Project is slowed down somewhat, as the feeling that Japan should be defeated swiftly in order to prevent a (Soviet) ground landing is more or less butterflied away.
The Asian war comes to an end with an invasion and occupation of Japan; the victorious allies agree to divide Japan into zones. A northern Soviet Zone, a western Chinese Zone, and a southern Western Allied Zone, (run jointly, (and inefficiently), by Britain, France and the USA). The occupiers agree unanimously agree to abolish the Imperial system, (the British voice some hesitance, arguing that the Emperor serves as a useful unifying force, but they are overruled).
In 1947, a bomb goes off in the Chinese Occupation HQ in Akita. The Chinese say the bomb was planted by a Japanese nationalist, supplied by a network that stretches across all three zones, and that the blast would not have occurred if the other two Occupiers manned their checkpoints correctly. Thus, the Chinese chose to close their checkpoints, and seal the borders of their zone. The Soviets and the Allies are annoyed, and warn China that if it doesn't reverse this move, it will be thrown out of the Occupation Agreement. China ignores the warning and instead starts terrorising its subject Japanese. Good to their warning, the other Occupiers expell China from their agreement, thus de-legitimising the Chinese occupation. There is talk of a joint invasion of the Chinese zone, but there is no mood for armed conflict, and, in any case, a more pressing urge to quell the low level insurgency that is still inflicting both zones.
Meanwhile, the issue of devolution to local power is becoming increasingly central. There is agreement that Japan should not remain divided, but no agreement whatsoever as to what form a united Japan should take. Eventually, the USSR and the Allies decide that the creation of a North, and South Japan will be more practical, and unification could then occur at a later date. North Japan, unsurprisingly, is modelled as a Socialist system - the USSR has had no problem in co-oping Japanese socialists to their side, with many from the South having chosen to head North to help the proletarian revolution, (the Japanese Leftsts are generally seen as being quite naive, and blind to the flaws of the occupying Soviets, but after nearly a century of the most horrific repression from their own governments and people, they can perhaps be forgiven for their optimism). The North Japanese government does all the standard communist things, collectivises farms and industries, disenfranchises the wealthy and destroys relics of bourgeois culture.
But at least it has a unified direction, which could not be said for South Japan. The Americans, British and French had trouble agreeing on much of anything, including what form the state should take. The Americans insisted on creating a strong presidency, while the British insisted on a strong legislature, and while the French insisted upon a strong, and activist judicial structure... the end result being an ultra-polyarchical system where everyone had veto power over everyone else, and where the power plays between the powers that be were played out on the governmental structure. Only four bills were passed in the first year since the introduction of the South Japanese constitution, the Occupation Police and their Japanese lackeys had little non-contradictory advice from their superiors, and so, when on the beat, often ended up just playing things by ear - usually erring on the repressive side, (afterall, many of these Occupiers fought the Japanese during the War, and many were not used to thinking of the Japs as innocent civilians, rather than bloodthirsty barbarians). Resentment amongst ordinary Japanese grew by the day.
The straw that broke the Joint Allied Occupation's back was the Suez Crisis of 1951, when Britain and France sought to invade Egypt, to reverse their decision to nationalise the Suez Canal, and were effectively forbidden from doing so by the USA. The outrage was palpatable, not only in the Franco-British military élites, but also amongst many ordinary citizens. As part of a wide range of responses, the French and British pulled out of Japan, leaving the Americans alone in what was becoming an increasingly violent uprising.
The Americans took the only sensible option and decided to withdraw too. Before they left, they invested dictatorial powers with a politician named Kuni Kuniyoshi, a sycophantically pro-American former nobleman.
Kuniyoshi's reign lasted just over eight months, eight bloody months. The cause of his downfall was an attempt made to steal a legislative election. By this point, the most popular party in Japan was the ultra-nationalist Japanese Peoples' Party, but in the election, Kuniyoshi's Liberal Party gained over 90% of the vote. South Japanese citizens revolted. Kuniyoshi was deposed and put on trial for treason. The JPP seized control and introduced a new law code, closely modelled on the pre-War Imperial system. Wisely, the JPP chose not to target the corporate and industrial élite, who had been so close to both the Americans and Kuniyoshi. Indeed, the South Japanese Revolution lead to an economic boom, as the morale of the Japanese workforce improved dramatically over night.
In early 1953, the South Japanese launched an invasion of Chinese Japan, by this time a thoroughly impoverished pariah state. Crucially, the invasion was launched the agreement of North Japan, (despite ideological differences, the Southerners were generally in favour of reunification, and were thus willing to forgive the Northerners their socialist indescretions - the Northerners were much less friendly however, they knew that the same Southerners who now offered the olive branch were the same bunch who would have oppressed them for the Leftist views just a single generation ago, but resentment for the South aside, the Northerners at least had the desire to live on a peaceful island).
The invasion went well, and was fought with the ferocity with which the Japanese were known for a decade previously. The Chinese were expelled, and the southern four fifths of the territory was incorporated into South Japan, the northern fifth was donated to the North. This wasn't so much an act of charity, as the northern-most part was the most urban, and thus the most damaged area, the North received the broken shell of the city of Akita for their limited troubles in the invasion. Akita was later rebuilt as a Stalinist megacity.
The expulsion from Japan triggered chaos in China, the Nationalist government, which had long been unstable, collapsed. Tibet and Xinjiang declared independence, and civil war was reignited in China. The USSR made efforts to support the communist forces in China, (eventually supporting a coup in Xinjiang and then annexing that country). Japan, meanwhile, sold arms to all sides, with the intention of keeping the war going on for as long as possible. The mood internationally was strongly anti-Imperialist, and so annexation of any part of China was out of the question, despite what many JPP politniks advocated. However, there were plenty of warlords in China willing to trade natural resources to Japan in exchange for arms. The Chinese Civil War in TTL ends up ressembling the Great Congo War of OTL, as many warlord factions rise and fall, trading raw materials for arms, as the surrounding smaller countries spread their influence and as ordinary civilians make up the bulk of casualties. By 1960, the death toll of the Chinese Civil War has exceded that of the Second World War.
Meanwhile, the Soviet Union moves closer to Japan. Japan's ability to provide economic growth while banning most political freedoms is admired, and many Soviet reformers wish to emulate it. Leonid Brezhnev, the Soviet leader from 1962, was particularly interested in emulating Japan, he placed a new emphasis on diplomatic relations and revitalised talks aimed at uniting North and South Japan. Brezhnev's internal economic reforms served to create an internal corporate élite through gradual, yet extensive privatisation. However, political reform was out of the question; like in Japan, there was a single party who's hegemony was not to be questioned.
In 1967, the Japanese Reunification Commission was founded. Japanese-Soviet relations were now strong enough that the USSR didn't need a puppet or a satellite separating them. The North Japanese old guard were reluctant, but the younger generation was more or less accepting, in any case, Moscow was feared/respected enough to be obeyed in these matters. In 1970, the Mikhalin-Eda plan was agreed, whereby sovereignty of North Japan would be transferred to South Japan, however, the Japanese Communist Party would remain in political power in the North, albeit as a subservient party to the Southern Japanese Peoples Party.
Meanwhile, other advances in Soviet-Japanese relations had taken place. A nuclear co-operation treaty had been signed, whereby the USSR would assist Japan in nuclear technology for both civilian and millitary purposes. And, more crucially, a peace plan for China. According to the peace plan, the warlords would be brought round the table, told not to expect any more weapon hand-outs, and to agree upon a post-Civil War government. The 20-year Civil War ended in 1974, as the new Government of National Unity, (a nation much smaller than OTL China), assembles in Nanjing.
The USSR and Japan both slowly democratise over the following decades.
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So there, an alliance between a Leftist USSR and a Rightist Republic of Japan. And yeah, I reread the thing about an alliance by 1950 about halfway through... at that point I was on a roll and just decided to run with it. Besides, the best responses to AH Challenges are the ones that ignore one of the criteria.
