December 1941 saw both defeat of the German army in its attempt to take Moscow and the entry of the United States into World War II. It became clear to German authorities that Germany would have to fight a long war.
Facing growing labor shortages and the ongoing need to produce armaments, machinery, airplanes, and ships to replace German losses, the SS established more SS-owned firms. It also signed contracts with state and private firms to produce goods and provide labor for the German armaments and related industries. A famous example of cooperation between the SS and private industry was the I.G. Farben company's establishment of a synthetic rubber plant in 1942 at Auschwitz III (Monowitz).
The incarceration of increasing numbers of people in the concentration camps assured the quantity of the labor supply even as the brutality of life inside the camps depleted the number of available laborers. The SS used gas chambers and other means to "weed out" prisoners who were no longer able to work.
During 1942–1944, hundreds of subcamps were established for each concentration camp. Subcamps were located in or near factories or sites for the extraction of raw materials. For example,
Central SS authorities tried to persuade camp commandants to focus their efforts on keeping the prisoners alive to serve the German war effort. However, few of the commandants took these instructions seriously. None were concerned about changing the murderous culture of the camps.
During the last year of the war, as the Germans retreated into the Reich itself, the concentration camp population (Jewish and non-Jewish) suffered catastrophic losses due to starvation, exposure, disease, and mistreatment. In addition, the SS forcibly evacuated concentration camp prisoners as the front approached because the Nazis did not want the prisoners to be liberated. Under SS guard, prisoners had to march on foot during brutal winter weather without adequate food, shelter, or clothing. SS guards had orders to shoot those who could not keep up. Other prisoners were evacuated by open freight car in the dead of winter.
During this period, the concentration camps were also sites of hideous and perverted medical experiments conducted on prisoners against their will and often with lethal results. For example, in Dachau, German scientists experimented on prisoners to determine the length of time German air force personnel might survive under reduced air pressure or in frozen water. In Sachsenhausen, various experiments were conducted on prisoners to find vaccines for lethal contagious diseases. At Auschwitz III, the SS doctor Josef Mengele conducted experiments on twins to seek ways of increasing the German population by breeding families that would produce twins.
These experiments were criminal and murderous. They were also based for the most part on bogus science and racist fantasy.
In 1944–1945, the Allied armies liberated the concentration camps. Tragically, deaths in the camps continued for several weeks after liberation. Some prisoners had already become too weak to survive.
According to SS reports, there were more than 700,000 prisoners left in the camps in January 1945. It has been estimated that nearly half of the total number of concentration camp deaths between 1933 and 1945 occurred during the last year of the war.