https://www.wsws.org/en/special/library/foundations-ger/10.html

73 - The KPD had been established as a response to the betrayal of social democracy. But it proved just as unable as the SPD to weld together the working class and lead it into a struggle against the Nazis. A ten-year campaign against “Trotskyism” had politically corroded the party and transformed its leadership into a willing tool of Stalin. It repeated all the opportunist and ultra-left errors, against which Lenin and Trotsky had fought ten years before, and hid its paralysis and fatalism behind radical phrase-mongering. Until 1933, Trotsky tried relentlessly to correct the wrong course of the KPD. His writings on Germany from these years, which fill two thick volumes, prove his genius as a Marxist and political leader. Banished to a remote Turkish island, forced to rely on newspapers and reports from political friends, Trotsky demonstrated an understanding of German events and their internal dynamics that remains unparalleled to this day. He foresaw the events clearly and precisely and developed a convincing alternative to the devastating course of the KPD. The KPD responded not with arguments, but with slanders, violence and the entire weight of the Moscow apparatus.

74 - At the heart of the policy of the KPD was the thesis of social fascism. From the fact that both fascism and bourgeois democracy were forms of capitalist rule, the Comintern drew the conclusion that there was no contradiction between them, not even a relative one. Fascism and social democracy were the same―in the words of Stalin: “not antipodes, but twins”―the social democrats therefore were “social fascists”. The KPD rejected any collaboration with the SPD against the rightwing danger and, in some cases, even went so far as to make common cause with the Nazis―for example, when it supported the referendum initiated by the Nazis in 1931 to bring down the SPD-led Prussian state government. Occasionally it called for “a united front from below”. But this was not an offer to collaborate, but an ultimatum to the SPD members to break with their party.

75 - Trotsky decisively opposed this form of vulgar radicalism. He recalled that Marx and Engels had protested fiercely when Lassalle had called feudal counterrevolution and the liberal bourgeoisie “one reactionary mass”. Now Stalin and the KPD were repeating the same error. “It is absolutely correct to place on the Social Democrats the responsibility for the emergency legislation of Brüning as well as for the impending danger of fascist savagery. It is absolute balderdash to identify Social Democracy with fascism”, he wrote. “The Social Democracy, which is today the chief representative of the parliamentary-bourgeois regime, derives its support from the workers. Fascism is supported by the petty bourgeoisie. The Social Democracy without the mass organizations of the workers can have no influence. Fascism cannot entrench itself in power without annihilating the workers’ organizations. Parliament is the main arena of the Social Democracy. The system of fascism is based upon the destruction of parliamentarianism. For the monopolistic bourgeoisie, the parliamentary and fascist regimes represent only different vehicles of dominion; it has recourse to one or the other, depending upon the historical conditions. But for both the Social Democracy and fascism, the choice of one or the other vehicle has an independent significance; more than that, for them it is a question of political life or death.”[3]

76 - Trotsky fought untiringly for a policy of the united front. This would have made it possible for the KPD to use the contradiction between social democracy and fascism to unite the working class, win the confidence of the social democratic workers and expose the social democratic leaders. In an article written at the end of 1931, entitled “For a Workers’ United Front Against Fascism”, he explained: “Today the Social Democracy as a whole, with all its internal antagonisms, is forced into sharp conflict with the fascists. It is our task to take advantage of this conflict and not to unite the antagonists against us.” One must “show by deeds a complete readiness to make a bloc with the Social Democrats against the fascists” and “understand how to tear the workers away from their leaders in reality. But reality today is―the struggle against fascism.” It was necessary to “help the Social Democratic workers in action―in this new and extraordinary situation―to test the value of their organizations and leaders at this time, when it is a matter of life and death for the working class.”[4]

77 - The refusal of the KPD to accept such a policy led to the German catastrophe.

I won’t say I agree with 100% of the analysis on that page but a lot of that last part of analysis seems completely spot-on to me. And, of course, how Trotsky predicted is exactly how it played out.