Julian Marchlevsky’s personality, responsabilities and activities at the time of the Russo-Polish war and during the peace negotiations show the blurriness of the institutional, professional and political boundaries between the revolutionaries, the military and the diplomats acting together at the multinational margins of the territory. The Polish Bolsheviks played a great role as intermediaries between Moscow and Warsaw. and their habitus and identity might have shaped the way the Bolsheviks negotiated with their adversaries.