When I think about German Expressionism the Isenheim Altarpiece (1506 -1515) by Matthias Grünewald comes quickly to my mind. I know it isn't exactly an Expressionist painting (I'm aware of the anachronism), but all expressionism (and some Surrealism too: Max Ernst, for instance) is there already.
One of the topics explored by Matthias Grünewald in his altarpiece is ergotism, as we can see in the polyptych's wing shown below:
The depiction of human pain (often psychological pain instead of physical agony) isn't the only theme explored by Expressionist painters (and it certainly isn't this artistic movement's monopoly), but it certainly is an important part of the aforementioned style. Otto Dix remembered Matthias Grünewald and the polyptich form (a tryptich with a predella: a gallery comic) when he painted Trench Warfare, 1929 - 1932, his own version of human suffering in WWI:
The religious connotation associated with Medieval and Renaissance retables was certainly intended (Otto Dix did many explicitly religious visual art during WW2 and after). We can find another religious connotation (but an ironic one this time) in another tryptych about WWI, the very mundane Metropolis, 1928:
That's the pessimistic, looking back in anger, reading, but there's also a warning to the Weimar Republic (applied also later to the Third Reich): the times were changing... The wind of change was coming from North America and we can see two indications of the new Weltanschauung in Metropolis: black culture and the Charleston (the vitality of the New World burying the decadent old one as seen in the center panel); women's new found freedom. Women's more practical clothing and short hairstyles are indication enough that another North American icon was influencing German roaring twenties' society: the flapper (the neue frau). (With an unprecedented public role during the war women wanted to keep it in the post-war social order; It was during the Weimar Republic that women in Germany earned the right to vote, for instance.)
Post-War Otto Dix was an increasingly realist painter. He was part of the Verists (a wing of the Neue Sachlichkeit; the New Objectivity). Dix was also persecuted by the Nazis: he was one of the painters shown in various anti-modern exhibitions: Reflection of Degeneracy (1933), Degenerate Art (1937), for instance. His "war against war," (the title of Ernst Friedrich's book of photographs Krieg dem Kriege, 1924) couldn't be well accepted by militarists.
Otto Dix put his great book Der Krieg in print in 1924 (Der Krieg is a cycle of twenty four offset plates published by art dealer Karl Nierendorf in Berlin - who also published a portfolio of fifty etchings - aquatint and drypoints).
Der Krieg isn't a powerful political statement like Metropolis. It's an Expressionist work tinged by verism already. It was done in the early twenties, after all, not during the first decade of the incredibly violent 20th century. These are impressive images which speak volumes about destruction and sufferance, but not much about the predators who are responsible for such disasters. War is, after all (as Clausewitz put it), a quest for power (i.e.: politics) and it should be judged as such...
In Der Krieg we witness horror of various kinds and, maybe, that's the book's hidden political meaning. Why do we see these things and do nothing? Why do we still believe in fantasies that force us to kill each other? Why can't soldiers see that they're closer (in all aspects) to their enemies in the war field than they are from those who send them kill each other?
This is a long shot though... This is me overinterpreting and rambling... but I can't prevent myself from thinking that the shell-shocked soldier in plate nineteen ("Verwundeter Soldat:" wounded soldier; see below) is directly addressing me, the viewer:
MoMA's Der Krieg collection
Der Krieg's historical and biographical context