A Solução Final

It is not known when Hitler formed the intention of the “final solution of the Jewish question” on the scale of the European continent. The conference in Wannsee on January 20, 1942 considered only the details of the undertaking: the methods for organizing the deportation and ensuring the cooperation of the civilian administration. Overall, the plans called for the murder of 11 million Jews living in Germany, the occupied territory, the states opposed to the Third Reich, and the allied and neutral countries.

The first killing center set up in occupied Polish lands was the camp at Chełmno on the Ner; Jews brought in from the ghettos in the Wartheland were being killed there from December 1941. Three more camps, somewhat larger, were opened at Bełżec, Sobibor, and Treblinka (in what was known as “Aktion Reinhard”) somewhat later, between March and July 1942.
In Auschwitz, the murdering of prisoners in gas chambers began even earlier, when 575 sick and disabled prisoners were sent to their deaths at the euthanasia center in Germany at the end of June 1941. At the beginning of September, the SS used Zyklon B gas in the cellars of block 11 to kill about 600 Soviet POWs and another group of patients from the camp hospital. Soviet POWs and Jews brought from Upper Silesia were killed in the gas chamber in crematorium I over the following months. It was probably at the end of March or in April 1942 that the Germans began killing sick prisoners and Jews in a provisional gas chamber in Birkenau (the so-called “little red house”). The tempo of atrocities increased in June and July 1942, with transports of Jews sent to Auschwitz being subjected to systematic “selections” during which SS doctors sentenced people classified as unfit for labor to death.
At the same time, the Germans set about liquidating the ghettos in occupied Poland. July 22, 1942, when the deportation of Jews from Warsaw to the death camp in Treblinka began, is regarded as a symbolic date. A decided majority of the Polish Jews were killed in a little over half a year, after which the SS began liquidating the Aktion Reinhard camps. However, the last great death camp—Auschwitz—remained in existence until the beginning of 1945. It was mainly Jews from Western and Southern Europe, from the liquidated labor camps, and the ghettos in Sosnowiec and Łódź, who died in the gas chambers there.

in en.auschwitz.org