"Doug, I agree that the "after Hitler, us" line ultimately proved to be self- destructive, but the German Communists certainly had a valid enough reason to distrust the Social Democrats after the latter crushed the Spartacist uprising."
Politics consists of more than this. Coalitions by definition consist of parties with reasons to distrust one another. Failure of the United Front was a catastrophe that defined the epoch. Lenin himself first proposed it, at a time when the SD's treachery was much fresher in mind. However, the SDs spurned it at that time, and again after the Third Period, so blaming the CP is at best one-sided and probably myopic also. The reason we hear so much more about "social fascism" than these other betrayals is because Trotskyism has so few refrains, and this is their most effective one for rallying the ranks.
(Liberals, by contrast, prefer to brandish the Molotov-Ribbentrop pact as their Stalinist betrayal of choice. When James Cannon proposed to Trotsky that the FI seize the opportunity to recruit CPers defecting after the pact with Hitler was signed, Trotsky admonished him not to, saying only the liberals were leaving; the real communists were as loyal as ever to the Party, gritting their teeth for the difficult days to come.)
Ernst Thaelmann wrote that the United and People's Fronts cost the Party many of its best and brightest, unwilling to unite with the murderers of their founding heroes.
Ken Lawrence