Kruk took part in founding the first labour Zionist grouping in his hometown Częstochowa prior to the Russian Revolution of 1905. When the revolution broke out, he was a major leader of the territorialist Zionist Socialist Workers Party in the city.
In 1906 he went into exile. Kruk travelled over most parts of Europe He studied Law at the University of Berne, obtaining a doctoral degree. During the First World War he and his wife, Roza Kruk, lived in London.
Kruk returned to Poland in 1918 and settled down in Warsaw. He became a leader of the Fareynikte party (into which the Zionist-Socialists had merged). In 1922 Kruk and the Fareynikte merged into the Independent Socialist Labour Party.
By 1927 the Independent Socialist Labour Party was in crisis. The question on cooperation with the communists divided the party. The party was split in two, one group led by Bolesław Drobner and another led by Kruk. Drobner's group merged into the Polish Socialist Party in 1928, whilst Kruk's group continued operate as the Independent Socialist Labour Party. After the split, Kruk took over Drobner's seat as one of the Polish representatives in the executive of the Labour and Socialist International. Kruk was a member of the executive between June 1928 and October 1930, sharing the seat with the Yugoslav socialist leader Živko Topalović (like Drobner had done as well).
- UNITED JEWISH SOCIALIST WORKERS' PARTY
- Czestochowa Poland
- Kadish, Sharman. Bolsheviks and British Jews: The Anglo-Jewish Community, Britain, and the Russian Revolution. London, England: F. Cass, 1992. p. 187
- Kowalski, Werner. Geschichte der sozialistischen arbeiter-internationale: 1923 - 19. Berlin: Dt. Verl. d. Wissenschaften, 1985. pp. 317-318
- Kowalski, Werner. Geschichte der sozialistischen arbeiter-internationale: 1923 - 19. Berlin: Dt. Verl. d. Wissenschaften, 1985. p. 284
- Marcus, Joseph. Social and Political History of the Jews in Poland, 1919-1939. Berlin: Mouton Publishers, 1983. p. 284