Killing on a mass scale using gas chambers or gas vans was the main difference between the extermination and concentration camps. From the end of 1941, the Germans built six extermination camps in occupied Poland: Auschwitz II-Birkenau, Majdanek, Chełmno, and the three Operation Reinhard camps at Belzec, Sobibor, and Treblinka II. Maly Trostenets, a concentration camp in the Reichskommissariat Ostland, became a killing centre in 1942. Gerlach writes that over three million Jews were murdered in 1942, the year that "marked the peak" of the mass murder of Jews. At least 1.4 million of these were in the General Government area of Poland.
The first major camp to be encountered by Allied troops, Majdanek, was discovered by the advancing Soviets on 25 July 1944. Treblinka, Sobibór, and Bełżec were never liberated, but were destroyed by the Germans in 1943. Auschwitz was liberated, also by the Soviets, on 27 January 1945; Buchenwald by the Americans on 11 April; Bergen-Belsen by the British on 15 April; Dachau by the Americans on 29 April; Ravensbrück by the Soviets on 30 April; and Mauthausen by the Americans on 5 May. The Red Cross took control of Theresienstadt on 4 May, days before the Soviets arrived.
Jewish prisoners were concentrated at Bergen-Belsen, hitherto not known as one of the worst camps; but in the chaotic final months of the war conditions were allowed to deteriorate catastrophically. When British troops came across the camp on 15 April 1945, they encountered 10,000 unburied corpses, a raging typhus epidemic and 60,000 sick and dying prisoners crammed into overcrowded barracks without food or water.
Pogroms occurred in several countries occupied by, or supportive of, Germany, attacks that were both encouraged by the Germans and carried out without their involvement. Thousands of Jews were killed in January and June 1941 in the Bucharest pogrom and Iaşi pogrom in Romania, a German ally. According to a 2004 report written by Tuvia Friling and others, up to 14,850 Jews died during the Iaşi pogrom. The Romanian military killed up to 25,000 Jews in Odessa, then under Romanian control, between 18 October 1941 and March 1942, assisted by gendarmes and the police. Mihai Antonescu, Romania's deputy prime minister, is reported as saying it was "the most favorable moment in our history" to solve the "Jewish problem". In July 1941 he said it was time for "total ethnic purification, for a revision of national life, and for purging our race of all those elements which are foreign to its soul, which have grown like mistletoes and darken our future".
The Dachau camp was a training center for SS concentration camp guards, and the camp’s organization and routine became the model for all Nazi concentration camps. The camp was divided into two sections — the camp area and the crematoria area. The camp area consisted of 32 barracks, including one for clergy imprisoned for opposing the Nazi regime and one reserved for medical experiments. The camp administration was located in the gatehouse at the main entrance. The camp area had a group of support buildings, containing the kitchen, laundry, showers, and workshops, as well as a prison block (Bunker). The courtyard between the prison and the central kitchen was used for the summary execution of prisoners. An electrified barbed-wire fence, a ditch, and a wall with seven guard towers surrounded the camp.
In the second phase of the evacuation, in April 1945, Himmler gave direct evacuation routes for remaining camps. Prisoners who were from the northern part of Germany were to be directed to the Baltic and North Sea coasts to be drowned. The prisoners from the southern part were to be gathered in the Alps, which was the location in which the SS wanted to resist the Allies. On 28 April 1945, an armed revolt took place in the town of Dachau. Both former and escaped concentration camp prisoners, and a renegade Volkssturm (civilian militia) company took part. At about 8:30 am the rebels occupied the Town Hall. The advanced forces of the SS gruesomely suppressed the revolt within a few hours.
In 1935 the Wehrmacht began to build a large military complex close to the village of Belsen, a part of the town of Bergen, in what was then the Province of Hanover. This became the largest military training area in Germany of the time and was used for armoured vehicle training. The barracks were finished in 1937. The camp has been in continuous operation since then and is today known as Bergen-Hohne Training Area. It is used by the NATO armed forces.