World War 1
Germany was wracked by communist revolts in 1919 (but because they weren't Russian backed, they failed miserably) - which ended with Rosa Luxemburg and Karl Liebknecht being shot in a Freikorps prison.
In 1924, the NSDAP under Adolf Hitler rose to power after a brief civil war. He then began forging alliances with Italy and Japan. His skillful diplomacy with Spain helped end military and economic restrictions after WW1.
With the Russian Federation/Soviet Union less eager to cooperate with the Germans and hence no Rapallo or Berlin treaties like OTL, they turned to Fascist Italy and Imperial Japan, which gave them the leeway to test new tanks.