With the outbreak of World War II in 1939, Nazi Germany instituted their extermination policies on an industrial level. Alex J. Kay recounts the organized mass murders in the book Empire of Destruction. At the beginning of the war, Nazis murdered approximately 70,000 institutionalized Germans with mental and physical disabilities by starving them, ending their medical care, or employing poison gas. These initial killings were considered a test run for the coming mass murder of the Jews: many of the same techniques and personnel were transferred to Eastern Europe.
Once Germany invaded Eastern Europe, the Nazis began large-scale systematic killings of ethnic Poles, prisoners of war, Catholic clergy, and of course Jews. A special mobile paramilitary killing force known as the Einsatzgruppen usually performed these executions. Eastern Europe took the brunt of the Nazi fury, as outlined in Waitman Wade Beorn’s book The Holocaust in Eastern Europe.
After Nazi occupation, the remaining Jews and members of other targeted groups were either deported to concentration camps or forced to relocate to ghettos, which were unsanitary and severely overcrowded. Food was scarce and disease was rampant given the living conditions. The second volume of The United States Holocaust Memorial Museum Encyclopedia of Camps and Ghettos, 1933–1945 provides an in-depth examination of the ghettos, describing over one-thousand wartime ghettos in Eastern Europe, including their creation and the fate of inhabitants.
As the war progressed, so did Hitler’s commitment to exterminating those he deemed “life unworthy of life” (lebensunwertes Leben in German). To further this end, SS leaders developed the Final Solution, a plan that outlined the systematic destruction of the Jews. More detail about the Final Solution can be found in the aforementioned The Origins of the Final Solution by Browning and Matthaüs.
The Einsatzgruppen could not perform executions quickly enough at this enhanced scale, so a more efficient method of mass killing was needed. In 1941, Nazi Germany implemented “Operation Reinhard,” the official plan to exterminate the two million Jews in Poland. This led to the creation of extermination camps with the sole purpose of industrializing killings. Yitzhak Arad’s The Operation Reinhard Death Camps examines these camps in detail. Victims were shipped to the camps, stripped of their belongings, executed in the gas chambers, and cremated on site with startlingly cold-blooded efficiency. Eventually, Jews from all over Europe were deported to these camps, including Jews from Bulgaria, the Netherlands, France, and Belgium. For a thorough analysis of all concentration and extermination camps, readers should consult KL: A History of the Nazi Concentration Camps by Nikolaus Wachsmann.
The Operation Reinhard Death Camps, Revised and Expanded Edition by 
