What would need to happen for Germany to become a strong, socialist/communist country (outside of Soviet influence, if the USSR exists), by the 1930s at the latest? (I realize this may require a pre-1900 POD)
In this TL, Germany can either win or lose WWI.
As OTL the spartacists have no chance. They need time to build up revolutionary support and for the moderates in the SPD to be discredited. They also need to get as many discharged former soldiers on their side as possible.
Maybe have the German government for some reason (economic straits maybe) refuse to give them any benefits and/or delay or reduce any backpay they had coming?
I'm not sure if that could work. There were too many monarchists and anti-democrats in Germany to keep even the republic really stable. Socialism / Communism would probably just create more problems.
Does Germany have to be a self-proclaimed socialist nation? Or can they just be one de facto?
There is a german AH book with that topic. ("Das Luxemburg Komplott")
The Groener - Ebert pact gets public knowledge and thats drives many SPD members and workers to the radical left.
The other POD is that Luxemburg and Liebknecht where rescued from death, so they lead the partly successfully revolution in Germany.
Finally the Reichswehr steps in and they win the German civil war and they establish a military dictatorship.
All in all, this was all a plot by the OHL to show the Entente powers that the Versailles treaty is to harsh and some terms needed to be scraped, so that the Reichswehr can fight the revolution.
What about if the 1933 elections go differently (possibly with several divided fascist parties, and maybe a more united left-wing popular front)? Could taking Hitler out of the picture lead to several competing fascist parties?
Sorry, I don't know that much German history.
And if the Spartacists succeed, would the British and French (or at least the US) intervene? Or would they be too occupied/exhausted from fighting Reds in Russia to worry about them in Germany? Could this lead to an earlier Allied withdrawal from Russia?