From Nikolaus Wachsmann: KL, A History of the Nazi Concentration Camps:
Which is part of the story. There’s also the broader view in Timothy Snyder: Bloodlands —
Fourteen million is the approximate number of people killed by purposeful policies of mass murder implemented by Nazi Germany and the Soviet Union in the bloodlands. I define the bloodlands as territories subject to both German and Soviet police power and associated mass killing polices at some point between 1933 and 1945. They correspond closely to the places were the Germans killed Jews between 1941 and 1945. In the east, more or less of Soviet Russia might have been included; but the existing line allows the consideration of the main German killing sites of the war as well as the western Soviet lands disproportionately struck by earlier Soviet terror. Though I discuss the western lands of today’s Poland, which belonged until 1945 to Germany, I do not include them in the bloodlands. This is to respect the difference between mass killing and ethnic cleansing. Hungary might arguably have been included, since it was occupied briefly by the Germans late in the war, after years as a German ally, and then occupied by the Soviets. After Polish and Soviet Jews, Hungarian Jews were the third-largest group of victims of the Holocaust. Romania, too, would have a kind of claim to belong to the bloodlands, since many of its Jews were killed and the country was occupied at the end of the war by the Soviet Union. Romania, however, was also a German ally rather than a victim of German aggression, and the murder of Romanian Jews was a Romanian rather than a German policy; this is a related but different history. Yugoslav citizens suffered many of the fates described here, including the Holocaust and mass reprisals; but the Jewish population of Yugoslavia was very small, and Yugoslavia was not occupied by the Soviet Union.These matters of political geography are debatable on the margin; what is not is the existence of a zone in Europe where Soviet and German power overlapped and where the tremendous majority of the deliberate killing of both regimes took place. It is indisputable, to state the point differently, that the contiguous area from central Poland to western Russia where Germans killed Jews covers the regions where all of the other major German and Soviet policies of mass killing had already taken place or were concurrently taking place—if not completely, then in very significant part. The purposeful starvation of Ukraine took place within the zone of the Holocaust. The purposeful starvation of Soviet prisoners of war took place within the zone of the Holocaust. Most Soviet and German shootings of Polish elites took place within the zone of the Holocaust. Most German “reprisal actions” took place within the zone of the Holocaust. A disproportionate amount of the shooting of the Stalinist Great Terror took place within the zone of the Holocaust.I use the term Molotov-Ribbentrop line to signify an important boundary running north to south through the bloodlands. This line (which appears on some of the maps) is the German-Soviet border as agreed in September 1939 after the joint invasion of Poland. It was significant for Polish citizens, since it marked the division between German and Soviet occupation policies. This line took on another meaning after the Germans betrayed their allies and invaded the Soviet Union in 1941. To its west, Germans were holding Jews in ghettos; to its east, Germans began to shoot Jews in very large numbers. The Holocaust began east of the Molotov-Ribbentrop line with shooting actions, and then shifted west of the Molotov-Ribbentrop line, where most victims were gassed.