In October 1939 Adolf Hitler ordered the widespread "mercy killing" of the sick and disabled in his Nazi Germany. The Nazi euthanasia program, known as "Aktion T4," was designed to eliminate "life unworthy of life." In the beginning the program was designed for newborns and very young children. Doctors were required to register all children up to three years of age who showed signs of mental retardation, physical deformity, or other symptoms included on a questionnaire from the Reich Health Ministry. Three medical experts would then make a decision on whether to allow the child to live, based only on the answers provided on the questionnaire. A unanimous decision to kill the child resulted in the issuing of a euthanasia warrant and the transfer of the child to a "Children’s Specialty Department" for death by injection or gradual starvation. If the vote was not unanimous, the child was kept under observation until three more experts could decide.

Hitler soon expanded his program and enlarged "the authority of certain physicians to be designated by name in such manner that persons who, according to human judgment, are incurable, can, upon a most careful diagnosis of their condition of sickness, be accorded a mercy death." Questionnaires were distributed to mental institutions, hospitals, and other institutions caring for the chronically ill. Six killing centers were established, including the well-known psychiatric clinic at Hadamar. It is within this program that the first experimental Nazi gassings took place. The patients were drugged and led into hermetically sealed gas chambers disguised as shower rooms. After the rooms were emptied of carbon monoxide, the bodies were taken away and cremated. The families were told that the cause of death was heart failure or pneumonia.

The huge increase in the disabled death rate and the smoke pouring from the killing centers aroused suspicion and fear. After Catholic Bishop Clemens von Galen spoke out against the Nazi euthanasia program, Hitler suspended Aktion T4. Secretly, the program continued, using drugs and starvation instead of the widespread gassings. In all, the Nazis involuntarily euthanized more than 100,000 sick and disabled people. The knowledge and experience from the euthanasia program was later used to create the huge concentration camps that were used to exterminate European Jews.