Originally posted by
Magnificate
Well, we can always fall back on the idea that the rest of Germany goes red and East Prussia manages to stay white due to its separation.
If it is a CP victory scenario, East Prussia is not separated from the rest of Germany - the Corridor, Danzig, etc. are still German. Therefore I do not believe that it is possible for East Prussia to go independent in case of German victory in WWI.
However if Germany lost... Imagine that the Kaiser refuses to abdicate and with some loyal army units, Prussian aristocrats etc. withdraws to East Prussia. Of course he is unable to continue the war, but he can not accept Germany as a republic. Moreover, he believes that sooner or later the republic will fall and German people will ask him (or his son) to return to power. So he waits...
Meanwhile, republican Germany finds a perfect scapegoat – the Kaiser. He and his aristocrats ruled Imperial Germany so they are responsible for loosing the war, milions of dead, territorial losses and all the suffering German people had to endure. There is no myth of the Dolchstoss or rather it is transformed into a leftist version – the German soldiers fought bravely, but all their sacrifices were wasted by the incompetent aristocratic generals and the Kaiser himself. Some Austrian corporal keeps shouting something about the Jews, but almost nobody listens to him.
The Allies are actually willing to allow for that situation to last. Divided Germany is weaker, which is good from French POV. The British are not so happy about it, but eventually they prefer to keep all German militarists safely isolated and unable to create any major troubles (or so they think). Moreover, the Allies are tired of war and will not send their armies to storm quite well fortified East Prussia.
With time, German Republic is getting better: German aristocrats, who followed the Kaiser are declared traitors and anything theirs that remained in Germany is confiscated and used to pay war reparations. Republican Germany is not exactly fond of Poland, but since nobody there wants the old regime back, it is possible to make some kind of deal with Warsaw, since Poland and German Republic have a common enemy. Poland promises to treat Polish Germans well and keeps its word since Poles too are worried about Prussia. In 1920, when Poles fight the Soviets, Prussian forces prepare to occupy Danzig and rest of the Corridor. It doesn't happen, however – the Kaiser did not want to cooperate with the communists nad waited for Poland to fall before sending his troops to protect the people from the bolsheviks. When Poland started wining the war, it was too late – a joint British-French naval and ground force secured Danzig – the Allies do not want the Kaiser to become too strong.
Eventually, we have Republican Germany in better political and economic situation, ruled by center and left parties. Polish-German relations are not very cordial, but there is some level of understanding and no open political or economical conflict (e.g. no customs war). Militaristic, aristocratic Prussia is considered the Bad Guy of Europe and is tolerated only as a potential ally against the USSR. The NSDAP desintegrates in late 1920s.