Franz Marc was born in 1880 and died in 1916 at the western front of WWI in France. Together with the Russian painter Wassily Kandinsky, he organized two expositions, in 1911 and in 1912. The group of participating artists called themselves “Der blaue Reiter” (“The Blue Rider”).
August Macke was born in 1887 and died shortly after the outbreak of WWI in 1914 at the western front in France. He participated in both expositions of “Der blaue Reiter”.
The theme of war is dominant in the early paintings of WWI veteran Otto Dix (1878-1969). During the Weimar Republic, he was in touch with the dadaists and experimented with techniques such as collages, movable pictures, and puppets. He was a founding member of the Dresden Secession group.
Max Beckmann (1884-1950) first was a member of the Berlin Secession but he cultivated an individual, with regards to some aspects neo-classical style. His participation in WWI as a paramedic unsettled him and explains his artistic dedication to the negative aspects of human nature.
An interesting blog about expressionism: Der Großstadt-Blog
To fathom the particular character of Expressionism, it is instructive to compare it to Impressionism: