Explanation: Despite their many differences, the KPD (Communist Party) and SPD (Social Democratic Party) of Weimar-era Germany actually managed a very successful period of cooperation against the conservative capitalists and reactionary monarchists in the early-mid-1920s.

… which ended with the ascension of the dictator Stalin in the USSR, and his associated bootlicker Ernst Thalmann in the German KPD. Marching orders from Comrade Stalin were that the Social Democrats and Democratic Socialists in the SPD were ‘social fascists’, and in some vague way even worse than regular fascists, and the only cooperation possible with the SPD was contingent on the SPD’s complete submission to the Soviet Union’s- sorry, the Germany Communist Party’s- goals.

After several years of this stonewalling and street fighting, the KPD heroically ensured the fall of the SPD and Weimar Germany as a whole to the fucking Nazis, and Stalin began ordering his proxies to cooperate with the Nazi regime.

Tankies blame the SPD for the death of Karl Liebknecht.