Birkenau was the largest of the more than 40 camps and sub-camps that made up the Auschwitz complex. During its three years of operation, it had a range of functions. When construction began in October 1941, it was supposed to be a camp for 125 thousand prisoners of war. It opened as a branch of Auschwitz in March 1942, and served at the same time as a center for the extermination of the Jews. In its final phase, from 1944, it also became a place where prisoners were concentrated before being transferred to labor in German industry in the depths of the Third Reich.
The initial mention of the idea of founding a camp in Brzezinka, a village near Auschwitz concentration camp, is connected with Heinrich Himmler’s first inspection of Auschwitz on March 1, 1941. The former Auschwitz commandant, Rudolf Höss, noted in his autobiography that Himmler issued a range of decrees during this visit about expanding the existing camp and employing the prisoners. One of the newly planned objects that Himmler listed on this occasion was a “camp for 100 thousand POWs.” There are however no documents from that period which would support the version given by Rudolf Höss and nothing indicates that Höss's account of the existence in March 1941 of the intention of building the Birkenau camp can be regarded as credible.
Despite extensive searches of the archival items, it has not proved possible to find any information indicating the existence before September 1941 of plans to prepare Auschwitz to receive large numbers of new inmates in the shape of POWs. In the construction documentation of KL Auschwitz there is mention (until mid-September 1941) about the beginning of any activities aiming at preparations to create the POWs camp.
An important document is a service note taken down on September 26, 1941 by the KL Auschwitz construction bureau after a telephone conversation with a representative of the Hauptampt Haushalt und Bauten SS. The information indicates the taking of a decision to create, adjacent to KL Auschwitz, a camp for prisoners of war with the capacity of 50,000. The note indicates the tremendous haste in the construction of the camp and the lack of any kind of previous planning. The word 'immediately' (sofort) is repeated no fewer than four times in this brief text.
See: The Origins of the Birkenau Camp in the Light of the Source Materials
A letter of September 27, 1941 from the Hauptampt Haushalt und Bauten SS to SS-Obersturmfuhrer Wolfgang Grosch, the Chief Plenipotentiary for the Construction of the POW Camp in Lublin contains an order to commence the building in Lublin and Auschwitz of two camps for Soviet POWs, each with a capacity of 50,000. The work is to be undertaken immediately and carried out as expeditiously as possible.
The first plans for the camp from autumn 1941 envisioned an initial capacity of 100 to 125 thousand prisoners. This concept underwent changes several times in 1942. The revised plans called for a doubling of capacity, to 200 thousand. The camp would be divided into four parts, often referred to as construction segments (Bauabschnitte), designated by roman numerals. The first segment was planned for 20 thousand prisoners, and the other three for 60 thousand each. The whole camp would cover an area of 175 acres.
The decision was made in 1941 to locate mass extermination facilities adjacent to the camp that was under construction in Birkenau—gas chambers for the mass killing of Jews brought to Birkenau as part of the Third Reich leadership’s plans for the complete extermination of the Jews of Europe. These gas chambers went into operation the following year.