So as some of you perhaps have noticed, I banned myself from this place, and thus was able to get some work done. I did indeed do this and thus did some preliminary research on the Japanese Marxist movement, a topic I have always had some latent interest in.
So the question is, how can we make Japan become Marxist/Communist instead of fascist in the prewar era? I want to focus on the prewar possibility because a PoD after the war is dubious at best and very limited in scope.
Here I present a summarized version of my views/opinions on the matter.
1) The Historical Conditions 1900-1920
OTL: Because Japan enjoyed much success regarding its economy and empire-building in this era, there weren't any major shocks to the system, despite there being some dissent and noticeable problems. For the nation as a whole, things were working out fine under the emperor system, Japan was happily industrializing,improving the living standard, expanding into Asia, and had won a war against a Western power, Russia.
In such a situation, there was not the societal desperation that normally accompanies the birth of a Marxist regime. If the system is more or less working, what need is there to overthrow it? So even though there were (and still are) poor people in Japan, and even though there were various left-wing intellectuals and attempts at organization and activity such as strikes, the formation of parties, etc, Communism never quite took off in Japan, having reached its height around 1920 and then crushed under the authoritarians in the 1930s.
TTL: Whatever grievances the Japanese may have had under the late Meiji and Taisho, it hardly needs be said that Japan as an imperial power was growing, spreading both its influence in Asia and developing the nation's industrial and economic strength at home, thus legitimatizing this government.
To destroy the successes of Japan in the Taisho era is thus to destroy its legitimacy, to show the nation that the imperial system no longer works. A good point for this turn is to have Japan's war against Russia go wrong, to have their designs on Korea and Manchuria repulsed. Thus, Japan will not be considered to have achieved the status of a great power like those of the West, and hence be excluded from various treaties and the diplomacy thereof. Because of all these setbacks, the Taisho rulers would try their luck once again in WW1, and instead of benefiting from it through increased trade, would once again be defeated by the Russians.
As a result, right-wing, imperialistic ideologies would lose support and the Marxists would be seen as a good alternative not only to workers and thinkers but to politicians and military men as well. Moreover, facing greater opposition, the government would react more oppressively against dissent and thus serve to unify the underground revolutionary movement. The left-wing movement would take on a new dimension as it attracts people in a broad demographic range, heralding the beginning of a clearly defined revolution of the proletariat against the old society.
2) Formation of Unified Japanese Marxism with relevance to factual and alternate Historical Conditions
OTL: There were two major "camps" for Marxism in Japan, the Rono-ha and the Koza-ha, touting respectively the idea of anarchist/Japan-specific revolution and a two-step revolution in accordance with a by-the-book Marxist interpretation of Japan's circumstances. In addition to the split nature of the Marxist presence in Japan, the uncertain political nature existent in the Taisho era made "correct" interpretation of the nation's circumstances difficult. Were, for instance, the bourgeois-liberal elements already developed to the point where illegal activity was no longer necessary? Was Japan a capitalist nation with feudal characteristics? What kind of revolution was thus in order? None of these questions could be answered effectively and without significant disagreement. Ultimately both groups were marginalized and crushed under the militarist regime.
TTL: As in the cases of Russia having suffered a disastrous WW1 campaign and of China and her four decades of political turmoil and international humiliation, meaningful Communist subversion manifests most easily under desperate circumstances.
Thus, for Marxism to take hold in Japan, we need the empire-building attempts of the Taisho to fail, as was mentioned earlier. These failures would lead to highly strained socio-economic relations domestically (like in Russia) and thus facilitate the formation of motivated underground socialist groups, especially as the government cracks down on legal opposition. The various Marxist theorists would be forced to agree that any revolution would have to be a determined, unconditional struggle against the feudalist/capitalist (state capitalism) rulers of Japan. Thus, a united Japanese Marxism-Leninism would evolve.
2a) Ideology: One of the reasons why I think it would be more interesting for Japan to become Communist before WW2 is because otherwise it would just end being a variant of the Soviet model, which is boring. SO let's explore the options
In OTL, most of the communists of Japan seemed to be rather pacifist, which given the odds they are facing is probably a recipe for not being able to take power. Also, as said before, they were split on basic theoretical ideas regarding how Marxism be interpreted for Japan's specific condition, and to add to this, the fact that the Comintern kept prescribing relatively ignorant interpretations did not help unify the movement.
Anyhow, for Marxism to become popular with the Japanese people, I don't think it can afford to present itself as being totally against order or tradition, but as an alternative to the current mode of economics and governance. The Japanese are very respectful of the Tenno-sei (emperor system) and thus if the Communists blatantly demanded its destruction I do not see this sitting well with the population as a whole.
As for the other details, I have to think much about them.
3) The Revolution (late 1920s)
I don't think the Japanese Revolution should be a civil war, but a political coup supported by elements of the military, with limited but decisive episodes of fighting, sort of like how Hitler came to power. A good year for the revolution, IMO would be 1926, since it was the end of the Taisho in OTL and is a good point for change.
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I have to go somewhere now, but I'll post the rest of what I planned regarding some basic international developments and post-revolution trends later tonight. In the meantime, please feel free to give your own opinions, knowledge, encouragement, etc. as I hope to have the gaps in my knowledge filled.
Oh, and my source, if anyone's interested, is a book called "Marxism and the Crisis of Development in Prewar Japan".