The camp system was one of the key instruments of terror, while at the same time providing necessary labour for the German war economy. Historians estimate that some 5 million Polish citizens (including Polish Jews) went through them.[1] Impartial scientific research into prisoner statistics became possible only after the collapse of the Soviet Empire in 1989, because in the preceding decades all inhabitants of the eastern half of the country annexed by the USSR in 1939 were described as citizens of the Soviet Union in the official communist statistics.[1]