Long before the Nazis ever came to power in Germany, the importance of eugenics in purging the race of the genetically unfit was being proposed. Not just in terms of sterilization, but execution of anyone deemed to be unworthy of life. Even after the Nazis came to power, it was recognized that the "problem" of dealing with the intellectually and mentally disabled could not be resolved in peacetime as there would be too great an opposition to doing what must be done. Despite the groundwork laid down by years of Nazi propaganda against the mentally disabled, it would take the outbreak of World War II in 1939 before the next step could begin in earnest.
It was in 1939 that, Action T4 (named for Tiergartenstrasse 4, the address of the building that headquarted the department that was to carry out the directive) began to carry out the eugenics program. It was actually voluntary at first. Parents would be asked if they wished their severely disabled children to be euthanized. A panel of doctors would assess individual cases of newborn children with various defects (real or suspected) and decide which children should be put to death. The voluntary element vanished quickly and parents were told that their children were being sent to centres where they would receive "special care". Afterwards, the parents would be told that their child had died of pneumonia . The bodies would be autopsied and tissue samples taken for medical research. As word began to spread, parents would refuse consent for their children to be taken and were often punished for it.
As Germany moved to a war footing, the program was expanded to include older children and adults. Lists of chronic patients were compiled from hospitals and sanatoriums (Jewish patients in particular were identifed). They were then transferred to special factilities where they could be killed. It would be in occupied Poland that T4 would first be implented on a broad scale with thousands of mental patients being killed by carbon monoxide gas in improvised gas chambers. The bodies were then burned in crematoria. It was the success of the program in Poland that led to the implementation of T4 across all of occupied Europe as the Germans advanced. Between 1939 and 1941, 75,000 to 250,000 people with intellectual or mental disabilities were executed by various means ranging from lethal injection to gas.
The architects of T4 were medical doctors with the training to carry out the "operational" aspects of the program (including Hitler's personal physician) but countless doctors and nurses also participated. As rumours of what was happening in the killing centres grew, many families tried to protect their loved ones by withdrawing them from hospitals and caring for them at home or arranging for doctors to "re-diagnose" them to avoid T4. It would be the opposition from church groups and other parts of society that led to Hitler formally cancelling T4 in 1941. While killings continued after 1941, they were less systematic and were absorbed into the broader Final Solution program for which T4 was an important first step.
In December 1946, the Doctors' trial was held to try 23 medical doctors and administrators for their role in "crimes against humanity" including human medical experimentation and their role in implementing T4. Seven were later executed and many others would receive long periods of imprisonment.