KPD (Communist) rally, with anti-SPD (Social Democrat) poster on the left, Weimar Germany, 1932

Huh. I didn't think the anti fascist flag dated back so far. I thought it was fairly new.

Many modern antifascists take the Iron Front flag of the same era instead
- I have no idea who’s downvoting this, somebody got really upset that you weren’t supporting stalinists

Anyone have an unbiased documentary about this stuff?

Really depends what you mean by unbiased. Almost all history is interpretive.
- I guess something that doesn't feel like a political ad. I like raw footage.

The line “first they came for the communists” from Martin Niemöller’s famous poem, goes back longer than just the Nazis.

After the fall of the Kaiser the SPD started working together with ultra rightwing shock troops (Freikorps) to subdue communists, the broader left and the labor movement. Gustav Noske (SDP) literally called himself the bloodhound (bluthond).

Lookup the death of Rosa Luxemburg and Karl Liebknecht for more fun facts about the SDP in that era.

"To subdue communists"
Cute way of saying the SPD resisting an armed attempt to overthrow a democratic government the KPD refused to participate in followed by the SPD cooperating with the KPD until Stalinization in the KPD changed the KPD's priorities from 'improving the position of the workers' to 'serving Soviet interests'
- Calling it a democratic government is quite a short cut. They were in the middle of a revolution, in the process of deciding what the future of Germany would look like, no elections had taken place. In the population there was huge support for dismantling the Prussian military institutions, but the SPD decided to not only keep the military as is but also to use it against their political opposition. I’m not saying the Spartakusbund were the good guys, I’m saying that the excessive violence of the SPD was foreboding of what was to come.