Yakov Hanecki was born in Warsaw, Vistula Land, Russian Empire, the son of Stanislav von Fürstenberg, a beer manufacturer of German descent, who had adopted Poland as his homeland. In 1896 he joined the Social Democracy of the Kingdom of Poland (SDKP - later the Social Democracy of the Kingdom of Poland and Lithuania (SDKPiL)) led by Rosa Luxemburg and her lover, Leo Jogiches. He moved to Germany in 1901 and studied in rapid succession at Berlin, Heidelberg, and Zurich universities. From 1902, he was a professional revolutionary, normally based in Cracow, under Austrian rule, organising the transport of illegal literature across the Russian border. In August 1903, as a member of the Main Administration of the SDKPiL, he was one of two Polish delegates to the Second Congress of the Russian Social Democratic Labour Party (RSDLP) in Brussels. The Congress later adjourned to London, under pressure from the Belgian police, and there the RSDLP split into its Bolshevik and Menshevik factions, but Hanecki and the other Polish delegate, Adolf Warski, did not make the journey to London, having failed to agree terms on which the RSDLP and SDKPiL could collaborate.