In April 1917 left-wing members of the Social Democratic Party (SDP) formed the Independent Socialist Party. Members included Kurt Eisner, Karl Kautsky, Eduard Bernstein, Julius Leber, Rudolf Breitscheild and Rudolf Hilferding.

For a time the Spartacus group worked with the I but in 1918 Karl Liebknecht, Rosa Luxemburg, Leo Jogiches, Paul Levi, Ernest Meyer, Franz Mehring and Clara Zetkin left to establish the Spartacus League.

At its congress in Halle in October 1920, a split occurred in the party. Left-wing members left to join the German Communist Party (KPD). The I continued until 1922 when it merged with the Social Democratic Party.