Russian social democrats had been split into numerous factions along political and ethnic lines since at least 1903 when the original divisions between the Bolsheviks and the Mensheviks arose.
Mezhraionka members were active in Petrograd during the revolution by seizing a printing plant and publishing the first leaflet calling for an armed uprising on February 27 O.S.
Although the Mezhraionka's original goal was to unite all Bolsheviks and Mensheviks in one party, the divisions over Russia's participation in the war proved to be too deep.
With the return of many anti-war social democratic émigrés from European exile in April to June 1917, the Mezhraionka was a natural place for them to join.
A number of prominent social democrats like Leon Trotsky, Adolf Joffe, Anatoly Lunacharsky, Moisei Uritsky, David Riazanov, V. Volodarsky, Lev Karakhan, Dmitry Manuilsky and Sergey Ezhov (Tsederbaum) joined it at that time.
The Mezhraionka, with about 4,000 members, merged with the Bolsheviks at the 6th Congress of the RSDLP in late July to early August 1917 in which both the groups formed a party that was formally independent of the Mensheviks.
One number was put out illegally in 1915, and publication was resumed in 1917, when it came out legally from June to August as the organ of the St. Petersburg Inter-District Committee of the United Social-Democrats (Internationalists).