"Progressives should not make the same mistake that Ernst Thälmann made in 1932. The leader of the German Communist Party, Thälmann saw mainstream liberals as his enemies, and so the center and left never joined forces against the Nazis. Thälmann famously said that ‘some Nazi trees must not be allowed to overshadow a forest’ of social democrats, whom he sneeringly called ‘social fascists.’
After Adolf Hitler gained power in 1933, Thälmann was arrested. He was shot on Hitler’s orders in Buchenwald concentration camp in 1944."
The background for the KDP’s uprisings is WWI. The war was incredibly destructive and pointless for every country in Europe. Before the war, the Second International (of which the SDP was a founding member) put out a manifesto that said:
However, once the war actually started, the SDP (along with many other social democratic parties in Europe) suddenly found all sorts of reasons to rally around the flag and support it unconditionally. The British socialists would point to problems in Germany under the Kaiser, the German socialists would point to problems with Russia under the Tsar, and each side would talk about how it’s not that they support the war, it’s just that they don’t want to lose. And so there was a failure across Europe (except in Russia, of course) to create domestic pressure to put an end to the war, and result was that it raged on until it had claimed 20 million lives.
It was only at the end of the war, when it was clear that Germany was going to lose regardless, that a revolution occurred, initially supported by both the SDP and the communists, which is what brought an end to the German Empire. During that uprising, the SDP and communists split over the direction of the country, and the SDP won and sent in the Freikorps to exterminated communist leadership. So when you talk about Thälmann trying to overthrow the government, I think it’s important to put that in the context that the government in question had come to power only 4 years prior by overthrowing the government - and that government would go on to last only 15 years before the Nazis were able to seize power through it. All of which is to say, it was a chaotic period, and there were reasons for the KDP to resent the SPD as well.
The tendency to force history into boxes defined by modern day politics loses a lot of that nuance. In contemporary American politics, there is no Second International. There is no Great War. There is no Sparticist Uprising. It’s bad enough when contemporary politics outside of the US are forced into the boxes defined by American politics, we don’t need to extend that throughout history.