By the time the Germans invaded Poland in September 1939, unleashing World War II, there were six concentration camps in the so-called Greater German Reich: Dachau (founded 1933), Sachsenhausen (1936), Buchenwald (1937), Flossenbürg in northeastern Bavaria near the 1937 Czech border (1938), Mauthausen, near Linz, Austria (1938), and Ravensbrück, the women's camp, established in Brandenburg Province, southeast of Berlin (1939), after the dissolution of Lichtenburg.

The chief of construction of Auschwitz II-Birkenau was Karl Bischoff, a competent and dynamic bureaucrat who, in spite of the ongoing war, carried out the construction deemed necessary. The Birkenau camp, the four crematoria, a new reception building, and hundreds of other buildings were planned and constructed.[42] Bischoff's plans, based on an initial budget of RM 8.9 million, called for each barracks to hold 550 prisoners. He later changed this to 744 per barracks, which meant the camp could hold 125,000, rather than 97,000.[43] The SS designed the barracks not so much to house people as to destroy them.[42] There were 174 barracks, each measuring 116 by 36 ft, divided into 62 bays of 43 sq. ft. The bays were divided into "roosts", initially for three inmates and later for four. With personal space of 11 sq. ft to sleep and place whatever belongings they had, inmates were deprived, Robert-Jan van Pelt wrote, "of the minimum space needed to exist".[44]
The Germans isolated all the camps and sub-camps from the outside world and surrounded them with barbed wire fencing. All contact with the outside world was forbidden. However, the area administered by the commandant and patrolled by the SS camp garrison went beyond the grounds enclosed by barbed wire. It included an additional area of approximately 40 square kilometers (the so-called “Interessengebiet” - the interest zone), which lay around the Auschwitz I and Auschwitz II-Birkenau camps.
After a fortnight we were able to use the money which we had brought in or was sent to us, and it became possible to complete and improve our diet from the canteen at our own expense. We were lined up in rows exactly in the order of our prison numbers, which had been sewed in black on white on our clothes. (This also is penitentiary procedure, for an inmate of the penitentiary is not an individual any more, but only a numbered being without a name.) Then we were led into the barracks where an S.S. man sitting behind a sort of teller's window disbursed the desired sum, up to fifteen marks a week. Somehow I must have transgressed against the military discipline in this procedure. I presume that I had my cap in the wrong hand. Instead of the money I received a violent blow on the back of my hand with a heavy stick which was lying next to the paymaster on his table, obviously for such purposes. Many weeks after this experience the back of my hand was still painful and swollen.
On April 27, 1940, Heinrich Himmler ordered the construction of a new camp near Oswiecim, Poland (about 37 miles or 60 km west of Krakow). The Auschwitz Concentration Camp ("Auschwitz" is the German spelling of "Oswiecim") quickly became the largest Nazi concentration and death camp. By the time of its liberation, Auschwitz had grown to include three large camps and 45 sub-camps.

Nazi, the informal and originally derogatory term for a party member, abbreviates the party's name (Nationalsozialist German pronunciation: [natsi̯oˈnaːlzotsi̯aˌlɪst]), and was coined in analogy with Sozi (pronounced [ˈzoːtsiː]), an abbreviation of Sozialdemokrat (member of the rival Social Democratic Party of Germany).[17][18] Members of the party referred to themselves as Nationalsozialisten (National Socialists), rarely as Nazis. The term Parteigenosse (party member) was commonly used among Nazis, with its corresponding feminine form Parteigenossin.[19]

The Auschwitz I main camp was a place of extermination, effected mainly by depriving people of elementary living conditions. It was also a centre for immediate extermination. Here were located the offices of the camp’s administration, the local garrison commander and the commandant of Auschwitz I, the seat of the central offices of the political department, and the prisoner labour department. Here too were the main supply stores, workshops and Schutzstaffel (SS) companies. Work in these administrative and economic units and companies was the main form of forced labour for the inmates in this camp.

With its sections separated by barbed-wire fences, Auschwitz II had the largest prisoner population of any of the three main camps. In January 1942, the first chamber using lethal Zyklon B gas was built on the camp. This building was judged inadequate for killing on the scale the Nazis wanted, and four further chambers were built. These were used for systematic genocide right up until November 1944, two months before the camp was liberated.
The Germans occupied Amsterdam in May 1940. In July 1942, German authorities and their Dutch collaborators began to concentrate Jews from throughout the Netherlands at Westerbork, a transit camp near the Dutch town of Assen, not far from the German border. From Westerbork, German officials deported the Jews to Auschwitz-Birkenau and Sobibor killing centers in German-occupied Poland.
If, up to now, we had interpreted our fate only as a privation of liberty, our experience changed rapidly. We had to jump down from the truck without the aid of a chair, and the request for a helping hand was denied with abuse. One of our comrades, an older man lacking the agility of youth, fell in this enforced jump and hurt the back of his head so badly that his skin had to be sewed with several stitches. Hardly were we standing on the ground when a pack of young men in S.S. uniforms, with yells and abuse, chased us to the other end of the large, inner, so-called inspection ground, which is surrounded by the barracks of the prisoners. Those who couldn't run fast enough were kicked.
Large segments of the Nazi Party, particularly among the members of the Sturmabteilung (SA), were committed to the party's official socialist, revolutionary and anti-capitalist positions and expected both a social and an economic revolution when the party gained power in 1933.[43] In the period immediately before the Nazi seizure of power, there were even Social Democrats and Communists who switched sides and became known as "Beefsteak Nazis": brown on the outside and red inside.[44] The leader of the SA, Ernst Röhm, pushed for a "second revolution" (the "first revolution" being the Nazis' seizure of power) that would enact socialist policies. Furthermore, Röhm desired that the SA absorb the much smaller German Army into its ranks under his leadership.[43] Once the Nazis achieved power, Röhm's SA was directed by Hitler to violently suppress the parties of the left, but they also began attacks against individuals deemed to be associated with conservative reaction.[45] Hitler saw Röhm's independent actions as violating and possibly threatening his leadership, as well as jeopardising the regime by alienating the conservative President Paul von Hindenburg and the conservative-oriented German Army.[46] This resulted in Hitler purging Röhm and other radical members of the SA in 1934, in what came to be known as the Night of the Long Knives.[46]
Many of the prisoners died in the concentration camps due to deliberate maltreatment, disease, starvation, and overwork, or they were executed as unfit for labor. Prisoners were transported in inhumane conditions by rail freight cars, in which many died before reaching their final destination. The prisoners were confined in the boxcars for days or even weeks, with little or no food or water. Many died of dehydration in the intense heat of summer or froze to death in winter. Concentration camps also existed in Germany itself, and while they were not specifically designed for systematic extermination, many of their inmates perished because of harsh conditions or they were executed.