Did the radical left cause the rise of Hitler?

It might be a tad heavy going in places, but When Insurrections Die is a classic imo. Short answer is that history is complicated and which way you decide to tell the story will depend greatly on what perspective you're coming from and what point you're trying to make, you can say it's the radical left's "fault" and they "should" have united with the SPD, or you can say it's the SPD's "fault" for siding with the Freikorps against the revolution at the end of WWI and making it impossible for people to work with them, or you can say it's the centrists' fault and that von Hindenburg "should" have done more to stop the nazis... etc etc.

Absolutely not.

This is the line usually pushed by Trots to contrast their United Front with the Stalinist Popular Front. It's often used now to argue for subsuming struggles under the control of the unions or other shitty recuperators.

The SPD had made any sort of Front with the KPD impossible when the SPD controlled Berlin cops massascred KPD May Day protestors in 1929.

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 Nazi militias.

From section 13 of https://libcom.org/library/nazism-and-working-class-sergio-bologna

Might be related to something Chomsky said in recent interview:

https://youtu.be/39902cn5lX8/?t=7m40s

Chomsky reignited it but its an old argument and hasn't really gone away. 2016 was full of this from American leftists urging people to support Clinton.

https://reddebreksbowl.blogspot.com/2016/11/2016-1933.html

It doesn't really pan out especially when it talks about elections, Hitler was made Chancellor before the election by German conservatives and the Nazis were given control of the Interior ministry and several state ministries for police. In Prussia Goring had already ordered the arrest and suppression of all communists, anarchists, socialists and Trotskyists and several other state police forces followed, and the SA had enrolled 50,000 of its members as a national force to assist the regular police.

The KPDs actions during the Weimar period were largely disastrous, but the SPD actively assisted in crushing the revolution of 1918 and in the process empowered the German far right including the leadership of the Nazi party. And they never lost their ability to inflict violence on their rivals in the labour movement. The two factions that enabled Hitler were German conservatives and the SPD leadership.