Bronski was a founder of the Zimmerwald anti-war movement, and a Polish delegate to the second Zimmerwald conference in Kienthal in April 1916. By then, he had become part of the Zimmerwald left, and an ally of the Bolshevik leader, Vladimir Lenin. By 1916, they were near neighbours in Zurich: it was Bronski who brought Lenin news in 1917 that the February Revolution had broken out in Russia. In June 1917, he reached Petrograd, where he joined the Bolsheviks, worked in the party's Agitprop department, and edited the Polish-language newspaper "Trybuna". After the Bolshevik Revolution, in November 1917, he worked for the state bank, and April supervised the first conference for prisoners of war (such as Bela Kun and Josip Broz Tito) who had converted to Bolshevism. From March 1918 until November 1918 he was the acting People's Commissar of Trade and Industry and until spring 1919, he was Deputy People's Commissar for Trade and Industry. In 1919, he was sent to Germany as an agent of Comintern, and addressed the founding congress the Young Communist International in a beer hall in Berlin in November 1919, and took part in a secret communist conference in Frankfurt and was a member of the Western Europe Secretariat. However he was very critical of Paul Levi's leadership of the Communist Party of Germany during the Kapp Putsch and was then recalled to Moscow.