Poplavsky was born in Imperial Russia, near Kiev. His family (father - Hilary) was ethnically Polish, and in his younger years he considered himself a Pole. He was a member of the Communist Party of the Soviet Union from 1930 onwards. Before the Second World War he attended the Frunze Military Academy (1935–1938) where he became an instructor of military tactics (1938–1939) but in February 1939 he was relieved after a false accusation and given a manager's job at a sovkhoz.