In 1915, after the SPD supported German involvement in World War I, she co-founded, with Karl Liebknecht, the anti-war Spartakusbund (Spartacist League). On 1 January 1919 the Spartacist League became the Communist Party of Germany (KPD). In November 1918, during the German Revolution she founded the Die Rote Fahne (The Red Flag), the central organ of the Spartacist movement.