The Bronski family were industrialists, who owned a cotton factory in Łódź. After leaving school in 1900, he emigrated to Munich to study at the Technical University of Munich, and later at the Munich University, and joined the Polish Progressive movement. In 1902, he joined the Social Democracy of the Kingdom of Poland and Lithuania (SDKPiL), led by Rosa Luxemburg and Leo Jogiches. On the outbreak of the 1905 revolution, in February, he moved to Warsaw, to work first as a propagandist for the SDPKiL, then as a member of the Warsaw city party committee. In 1906, he edited the party newspaper Czerwony Sztandar ("Red Flag"). He was arrested in Lublin in autumn 1906, and held in prison until the end of 1907. After his release, he emigrated to Switzerland, and joined the Social Democratic Party of Switzerland. When the SDKPiL split in 1911, he joined the 'rozlamovist' faction, led by Jacob Hanecki and Karl Radek.
