revolutionary socialist group active in Germany from autumn 1914 to the end of 1918. It was officially founded in 1916 by Karl Liebknecht (Liebknecht, Karl), Rosa Luxemburg (Luxemburg, Rosa), Clara Zetkin (Zetkin, Clara), and Franz Mehring (Mehring, Franz). The name derived from their illegally distributed pamphlets Spartakusbriefen (Spartacus Letters). The league developed as an offshoot of the Social Democratic Party, among elements who violently opposed Germany's role in World War I and who called for a socialist revolution.

The Spartacus League was transformed into the Communist Party of Germany at a party congress held from Dec. 30, 1918, to Jan. 1, 1919. Meanwhile, the Spartacists had encouraged demonstrations in December that led to the abortive Spartacus Revolt in Berlin in January 1919. On January 15, Luxemburg and Liebknecht were arrested and murdered in Berlin by members of the conservative Free Corps ( Freikorps), who had seized control of the city's police presidium.