Fascism in Germany could have been smashed, had a proper revolutionary policy been pursued by the two "Marxist" parties which together had a majority during the election that drove Hitler to power: the Social Democratic Party (SPD) and the Communist Party (KPD).
Together they had hundreds of thousands of armed workers in revolutionary militias, and the decisive support of the working class remained with them. Hitler and the Nazis actually decreased their vote from the previous elections. Instead, the KPD put forward the 3rd International's suicidal slogan of "Social Fascism" and the "Third Period". The idea was that Capitalism went through three periods, one was the revolutionary period culminating in the 1917 revolution, the second was a period of lull and reassendance of Capitalism, and the third was the period of terminal decline of Capitalism.
The revolution was now, and the Social Fascists (Social Democrats), were the real threat, not the Nazis. What difference does it make if a Social Democrat is chief of police, or a Nazi? This ultra-left insanity led to the "Red Referendum", where the KPD campaigned together with the Nazis to remove an SPD governor. Even worse, KPD activists, on party orders, set out to disrupt SPD party and union meetings, with the spectacle of Nazis and KPD activists ganging up on SPD workers and thrashing the "real Fascists": the vast majority of workers, who still clung to their reformist leadership, despite all the betrayals. This fratricidal warfare did not give the working class a clear lead on how to face the rising mortal threat of Fascism, instead it only split and disoriented the workers at the very moment where they needed maximum unity and decisive action against the class enemy.
Instead of shaming the leadership, by calling for a working-class united front, with the KPD's and SPD's militants together beating up Nazis and smashing them, the KPD alienated the working class by a lunatic and sectarian policy that led directly to Hitler's rise to power. The armed working class outnumbered Hitlers' supporters, and could have defeated him even on the eve of his rise to power. Instead, in Hitler's own words, he took power "without shattering a window-pane". The KPD's response? "After Hitler, our turn". It was a victory for the KPD! Having led the working class to the slaughterhouse, the KPD had one last chance to call for a generalized resistance movement. A mass protest, a general strike, anything. But they were too blind to even admit anything was wrong. The blood is on their hands more than anyone else.
The failure lies not just on the leaders of the Second, but of the Third International first and foremost. Their near-sighted and lost approach to the international revolution squandered the greatest revolutionary period humanity has ever seen. The early hope and revolutionary spirit that the shining example of October 1917 gave to the German and world working class, following defeat after blundering defeat, gave way to demoralization and despair on the part of the working class, and depraved bloodlust on the part of the working class.
The failure of the international revolution led inextricably to the victory of international Fascism, defining the darkest period in modern history.