After the German invasion of Poland that marked the start of World War II in 1939, the Soviet Union invaded and annexed eastern parts of the Second Polish Republic. In 1940 the Soviet Union occupied Estonia, Latvia, Lithuania, Bessarabia and Bukovina. Hundreds of thousands of Polish citizens and inhabitants of the other annexed lands, regardless of their ethnic origin, were arrested and sent to the GULAG camps.
Approximately 300,000 Polish prisoners of war were captured by the USSR during and after the Polish Defensive War. Almost all of the captured officers and a large number of ordinary soldiers were then murdered (see Katyn massacre) or sent to GULAG. Of the 10,000-12,000 Poles sent to Kolyma in 1940-1941, most POWs, only 583 men survived, released in 1942 to join the Polish Armed Forces in the East. Out of Anders’ 80,000 evacuees from Soviet Union gathered in Great Britain only 310 volunteered to return to Soviet-controlled Poland in 1947.
According to the official data, the total number of sentences for political crimes in USSR in 1939-41 was 211,106. During the war, Gulag populations declined sharply, as a consequence of the mass releases of hundreds of thousands of prisoners who were conscripted and sent directly to the front lines and a steep rise in mortality in 1942–43. In the winter of 1941 a quarter of the Gulag’s population died of starvation. 516,841 prisoners died in prison camps in 1941-43.
In 1943, the term katorga works (каторжные работы) was reintroduced. They were initially intended for Nazi collaborators, but then other categories of political prisoners (for example, members of deported peoples who fled from exile) were also sentenced to “katorga works”. Prisoners sentenced to “katorga works” were sent to Gulag prison camps with the most harsh regime and many of them perished. Other gulag inmates were sentenced to Red Army penal battalions, where they performed extremely hazardous duties such as spearheading Russian offensives, often serving as tramplers, persons who marched or ran over German minefields to clear them for successive Red Army infantry formations.
A total of 427,910 served in penal units from September 1942 to May 1945. These totals should be viewed in comparison to the nearly 34.5 million men and women who served in the Soviet armed forces during the entire period of the war.