In the summer of 1901 he emigrated to Berlin, where he joined the Iskra assistance group. On his return, he joined the Russian Social Democratic Labour Party in 1902 and went to London for their Second Congress in 1903. Dan aligned himself with Julius Martov who wanted to have a larger party of activists, rather than Vladimir Lenin's conception of a smaller party of professional revolutionaries. Dan helped Martov form the Mensheviks, returning to Russia in 1912. He was an active member of the irregular freemasonic lodge, the Grand Orient of Russia’s Peoples. From 1913 he worked legally in Russia. Editor of the newspaper Golos Sotsial-Demorata and was a member of the Menshevik Central Committee. In 1915 he was arrested and exiled to Siberia. He was pardoned and mobilized as a military doctor with the outbreak of WWI.