In 1900, he joined the Social Democracy of the Kingdom of Poland and Lithuania (SDKPiL), led by Rosa Luxemburg and Leo Jogiches.

[4] In 1911, he joined the rozlamovists, a group of mainly younger SDPKiL members, led by Yakov Ganetsky, who opposed Jogiches' leadership methods, and who were close to Vladimir Lenin and the Bolsheviks.

[5] At the time of the February Revolution, Unszlicht was in exile in Siberia, where he was elected a member of the Irkutsk soviet, and joined the Bolsheviks.

In April, he led the defence of Wilno (Vilnius) as it was being overrun by the Polish army, commanded by Józef Piłsudski.

[8] He was transferred to the post of chief of supply for the Red Army, although Leon Trotsky, the People's Commissar for War, regarded him as "an ambitious but talentless intriguer" who had been placed there to undermine him.

Two weeks later, the head of the NKVD, Nikolay Yezhov told a plenum of the Central Committee of the Communist Party that the police had uncovered a "Polish Military Organisation" (POV) of spies who had infiltrated the USSR by posing as political émigrés, and named Unszlicht as its leader.