In 1939, Adolf Hitler signed an order leading to the eventual slaughter of an estimated 300,000 physically disabled and/or mentally ill children and adults in Germany and its annexed territories. The “Euthanasia Program” — or Aktion T4 , as it came to be known — consisted of the systematic killing of human beings deemed not worthy of life or care: “useless eaters,” as they were called by some. The name “Aktion T4” was borrowed from the street address of the office that administered the program: Tiergartenstrasse 4. The scheme was initially built on deception: families were told that their children...