After Adolf Hitler gained power in 1933
WHO GAVE HITLER POWER MOTHER FUCKER?
Nobody in history has been more vindicated than Ernst motherfucking Thälmann. A vote for a Social Democrat is a vote for fascism now just as it was then - and the Democrats aren’t even that!
The SPD was joining with the right to crush workers’ movements long before the election of 1933. If the KPD had joined with them in a coalition it would have represented the KPD abandoning the German workers, and events from then on would have played out largely the same because the Social Democrats enthusiastically went after the Communists along with the Nazis, and it was once the Communists were taken out that the Nazis turned their ire towards the Social Democrats.
The only ways Wiemar Germany turns out different is if a) the SPD joins, rather than represses, the Spartacist Uprising, or b) the KPD manages to take control before being destroyed. It isn’t about being perfect, it’s about preventing the forces of reaction from having a foothold in the movement.