A good refutation of the ‘KPD is to blame for the Third Reich’ myth is Sergio Bologna’s Nazism and the working class, which focuses more on class analysis (!!!) than blaming ‘the left’ or the Comintern or Moscow or specifically Joseph Stalin for almost everything that went wrong. (It’s also a big surprise coming from libcom.org—of all things—hilariously enough.) Anyway, our theory noob here claims that

the KPD […] rejected several offers from the SPD to enter coalition to stop the rise of Nazism

While there is some truth to this, when we actually examine the relations between the SPD and the KPD in context, as Nazism and the working class does, the reasons for this refusal were more complicated than how she presented them. She also claims that the

KPD and NSDAP also essentially collaborated during the late 1920s and early 1930s to undermine Weimar democracy both as a matter of street violence and using institutional means

By which she meant

in history books you too often find the thesis that the Nazis and Communists went side by side to fight against the institutions of Weimar, and you frequently find reference to the two episodes in which they found themselves in a united front against the Socialist Party: the public transport strike in Berlin in Autumn 1932, and the referendum against the Prussian government under Otto Braun; you almost never hear of the physical clashes which took place between proletarians organised by the KPD and the [Fascist] gangs.

What was wrong with this Prussian government (or ‘Weimar democracy’) under Braun? Sergio Bologna provides us with a clue:

Now, Prussia was governed not so much by the Social Democratic party as such, as by some of its more prestigious exponents. They had considerable power, and they were located on the extreme Right of the party.

The key man in Prussia, for many years prime minister of the Prussian government, was Otto Braun, a man of open and declared authoritarian tendencies, who saw the role of social democracy as being in maintaining law and order, in the untouchability of the state bureaucra[c]y, and in a corporative partnership between trade unions and big capital.

In the words of Theodor Eschenburg, the author of a fine book on the problem of “ungovernability” in the Weimar Republic, he was in favour of a “recallable dictatorship”. Otto Braun's principal collaborator was for many years Albert Grzesinski, who was Minister of the Interior in Prussia, and from 1930 was also head of police in Berlin.

We should not forget that during this period the Social Democrats had considerable powers in the area of law and order, because in 1928 one of their number, Carl Severing, was appointed Minister of the Interior of the Reich. The SPD took advantage of this to institute an extremely efficient reorganisation of the police, with the principal aim of setting up a special corps to prevent Bolshevik disturbances and uprisings.

Unfortunately they were not equally efficient and motivated in preventing and repressing [Fascist] gangsterism. The situation inevitably aggravated the historic fracture between Social Democrats and Communists that had already existed since the murders of Rosa Luxemburg and Karl Liebknecht — a fracture which experienced a particularly acute moment — a point of “no return” — in the events of Mayday 1929.

[…]

Mayday 1929 in Berlin fell in an atmosphere that was particularly tense, due partly to the onset of economic crisis and partly to the onset of a crisis of the political system.

The police chief in Berlin, a Social Democrat by name of Zorgiebel, had already banned all public demonstrations in Berlin in December 1928. In March 1929 he extended the ban to the whole of Prussia, and then renewed the ban specifically for Mayday 1929, asking the trade unions to abstain from public demonstrations and to organise only indoor meetings.

The Communists, however, decided to challenge the ban and to demonstrate in the streets. The Social Democratic trade unions and the SPD organised their Mayday events in theatres, association offices etc. The Communist slogan was: "We do not accept the ban. We shall demonstrate in the streets, and if the police try to attack we shall call a general strike for the next day." And so it was to be.

The police, as has been shown from research in police archives, mounted a deliberate attack, organised by special anti‐subversion units. There were violent clashes, which spread to include workers who were coming out of the indoor meetings of the Social Democratic trade unions. The Communist Party called a general strike for the following day, but despite pressure from many militants did not distribute weapons; nevertheless, in the quarters of Neukolln and Wedding the barricades went up and the police had to lay siege to the areas for three days before they were able to restore order.

The final balance was extremely heavy: thirty people dead, all of them demonstrators; 200 wounded; 1,200 people arrested, of whom 44 were kept in custody by the police. The Prussian Minister of the Interior seized this opportunity to ban the mass organisations of the Communist Party.

These events brought about an unhealable fracture between Communist militants, and the Social Democratic party and its organisations. Oral history research has shown that in the memory of proletarian militants (not only communists) this was a turning point, a “point of no return” in their remembrance of their total alienation from anything to do with the SPD.

Whereas the killings of Rosa Luxemburg and Karl Liebknecht might possibly have been attributed to the Freikorps and not purely to Noske's policies, the blame for the repression of Mayday 1929 in Berlin lay squarely at the door of Social Democratic ministers and functionaries. This trauma split the working class down the middle, right on the eve of the final clash with the [Fascist] militias.

(Emphasis added.)

With all of this in mind, it should not be hard at all to understand why the KPD would turn down the SPD’s late offers for a ‘coalition’ (probably under some very important terms and conditions that somebody neglected to mention, I suspect) against fascism. Given that the succdems were so lazy at suppressing fascism, I really doubt that a coalition would have made an enormous difference anyway.

As for why the Fascists wished to suppress German Social Democracy, hopefully it goes without saying that their reasons were very different from those of the Communists, which probably explains why their collaboration — if you’d call it that — was so sporadic and ultimately meaningless. It just goes to show that being right for the wrong reasons is hardly better than being wrong to begin with.