After the Serbian victory in the First World War by the end of 1918 Montenegro enters the political union with Serbia under the Karadjordjevic dynasty. It is important to stress that this resolved the long-standing dynastic rivalry between the two Serb royal families: the Petrovic Njegos family of Montenegro and the Karadjordjevic family of Serbia. The decision was taken at the historical Great people's council of Podgorica on 26th of November 1918. The main decisions reached at the event were: 1.) The unification of Serbia and Montenegro, 2.) dethronization of King Nikola I Petrovic and the acceptance of the Karadjordjevic dynasty. Although most Montenegrins (the Whites) were in fervor of these decisions, one part of King Nikola's supporters (the Greens) wished to preserve the Petrovic Njegos dynasty, and also wanted the union with Serbia but under completely different terms (a federal union and not a centralized one). These groups got their names according to the color of the session's voting sheets.

With support of Italy the Greens organized the Christmas Uprising in 1919 with the aim to bring the Petrovic Njegos dynasty again on the Montenegrin throne. The rebellion failed, the rebels fled to Italy, later returned and in June started unsuccessful guerrilla war. In traces the Greens resisted till 1926 and after that their struggle was represented in political program of Montenegrin Federalist Party: Montenegro, Serbia and other Yugoslavs in federal state.