From 1913 Rau also was active in the labour movement. In that year he joined the metal workers' union ("Deutscher Metallarbeiterverband") and a social democratic youth group in Zuffenhausen. During the following years, which saw the beginning of World War I, Rau's youth group, whose leader he became in 1916, was significantly influenced by the left wing of the Social Democratic Party of Germany (SPD). The leftists considered the war a conflict between "imperialist powers". A few local members of a far left SPD group, among them Edwin Hoernle and Albert Schreiner, who later became well-known members of the Spartacus League ("Spartakusbund"), visited the youth group in Zuffenhausen and gave lectures. In 1916, Rau joined the Spartacists as well and became a co-founder of their youth organisation. In accordance with the politics of the Spartacists, in 1917 he joined the left-wing Independent Social Democratic Party of Germany (USPD) and in 1919 the Communist Party of Germany (KPD), which had been founded mainly by members of the Spartacus League.