Schmidt-Rottluff was a member of 'Die Brücke', not 'Blaue Reiter'.
Lash wrote:What was going on socially and politically in the 20's in Germany? I don't know much about that period.
Both, Die Brücke and the Blaue Reiter group were founded long before the 20's (several of the main members actually died in WW1).
Is your question realted to art or just asked generally?
(In the 20's, there was the Weimar Republic. the first democracy in Germany. And the social situation was quite similar to elswhere in Europe.)
After WW1, "Die Neue Sachlichkeit" (e.g. paintings by Otto Dix and George Grosz) showed the disillusionment following the war.