SS leader Heinrich Himmler orders the expansion of the concentration camp of Auschwitz to accommodate a large influx of prisoners, (c. 130,000) who will supply forced labor to German factories in the area. Auschwitz ultimately grows into a massive complex of over 40 subcamps, including the largest Nazi killing center established at Auschwitz-Birkenau in September 1941.
Moving behind German military lines, Einsatzgruppen--"mobile killing squads"--of Nazi military and police units under the command of Reinhard Heydrich carry out systematic massacres of Jews as the Nazis advance eastward, with the intention of annihilating the Jewish population of the Soviet Union. Between 1.5 and 2 million Jews are shot to death in what is known as the "Holocaust by bullets." Among the many mass atrocities committed by the Einsatzgruppen in 1941: about 10,000 Jews are murdered near Vilna in July; over 160,000 Jews are massacred in Romania in July and August with the help of Romanian troops. In September, approximately 34,000 Jews are massacred at Babi Yar in Kiev and 27,000 in Zhitomir; in October, 48,000 Jews are massacred near Odessa, and 25,000 Jews are murdered in Riga in November and December.
The Jewish ghetto in the Polish city of Lublin is sealed, cutting off more than 34,000 Jews from the outside world.
Germany's invasion and occupation of Yugoslavia and Greece brings hundreds of thousands of Jews under Nazi rule.
A Jewish ghetto is established in the medieval city of Krakow, capital of the Nazi Generalgouvernement. Approximately 20,000 Jews are sealed inside, condemned to forced labor and living in unspeakable conditions.