May 1939: Hitler — as per a request from the boy's father — agrees to authorise the killing of Gerhard Kretschmar, a child born blind and with limbs missing. After the 5-month-old child's death in July, Hitler quickly initiates a Children's Euthanasia program and upwards of 5,000 youngsters are killed due to "defects."
October 1939: Hitler signs the following memo, backdated to September, and the euthanasia program becomes official. Now named Action T4, the program no longer discriminates with regards to age and all persons, young and old, can now be killed should they be "unworthy of life." As noted in the memo, Karl Brandt and Philipp Bouhler are handed the reigns and over the next few years will oversee the murder of approximately 200,000 people. Action T4's need to kill humans in such vast numbers will also result in the introduction of "death by gas" to the Nazi regime.
Transcript and translation follow.

Transcript
BERLIN, DEN 1.Sept.1939.
Reichsleiter Bouhler und
Dr. med. Brandt
sind unter Verantwortung beauftragt, die Befugnisse namentlich zu bestimmender Ärzte so zu erweitern, dass nach menschlichen Ermessen unheilbar Kranken bei kritischster Beurteilung ihres Krankheitszustandes der Gnadentod gewährt werden kann.
(Signed, A. Hitler)
Translation
BERLIN, 1.Sept.1939.
Reichsleiter Bouhler and
Dr. med. Brandt
are instructed to broaden the powers of physicians designated by name, who will decide whether those who have - as far as can be humanly determined - incurable illnesses can, after the most careful evaluation, be granted a mercy death.
(Signed, A. Hitler)