However, the agreement did not last long as Haase, Wilhelm Dittmann and Emil Barth left the council on 29 December 1918 to protest the SPD's use of military force during the sailors' uprising in Berlin.

At the same time, the Spartakusbund, led by Rosa Luxemburg and Karl Liebknecht, separated from the USPD in order to merge with other left-wing groups and form the Communist Party of Germany (Kommunistische Partei Deutschlands, KPD).

[4] Whilst Däumig and Stoecker agreed with the International's 21 conditions of entry, Crispien and Dittmann opposed them,[4] leading to a controversial debate over joining the Comintern to break out in the USPD.

Ultimately, the proposition to join the Comintern was approved at a party convention in Halle in October 1920 by 237 votes to 156,[5] with various international speakers including Julius Martov, Jean Longuet and Grigory Zinoviev.

Following the assassination of foreign minister Walther Rathenau by right-wing extremists in June 1922, the two parties' factions in the Reichstag formed a common working group on 14 July 1922.

Elements of the USPD continued as an independent party led by Georg Ledebour and Theodor Liebknecht, who refused to work with the SPD, but it never attained any significance again.