Polish gentiles who rescued Jews had several obstacles to overcome. The most serious were the German laws against protecting Jews and the corresponding obligation to identify those who violated these laws. On 15 October 1941, the Nazis introduced a widely publicized law making unauthorized Jewish movement outside ghettos a crime punishable by death. The same punishment applied to gentiles who helped Jews escape. Transgressions were promptly followed by executions that were also widely publicized.
With the beginning of war and the organized murder of "undesirable" non-Jewish groups among the German population in the so-called Euthanasia program, hazy declarations of intent and expectation from the top leadership – most prominently Hitler's Reichstag statement of January 30, 1939, that a new world war would bring about "the annihilation of the Jewish race in Europe" – provided legitimization and incentive for violent, on occasion already murderous measures adopted at the periphery that would in turn radicalize decision making in Berlin. Heydrich's Schnellbrief to the Einsatzgruppen commanders in Poland dated September 21, 1939, on the "Jewish question" refers to secret "planned total measures" (thus the final aim) ("die geplanten Gesamtmaßnahmen (also das Endziel")); nevertheless, most Holocaust historians today agree that at the time this solution was still perceived in terms of repression and removal, not annihilation. The more frequent use of the term Final Solution in German documents beginning in 1941 indicates gradual movement toward the idea of physical elimination in the context of shattered plans for large-scale population resettlement (including the "Madagascar plan") and megalomanic hopes of imperial aggrandizement in Eastern Europe. American scholar Christopher Browning notes that "a 'big bang' theory" fails to adequately describe German decision making; instead, the process was prolonged and incremental, driven by "a vague vision of implied genocide."
On September 21, 1939, Reinhard Heydrich ordered the establishment of the Judenräte (“Jewish Councils”), comprising up to 24 men—rabbis and Jewish leaders. Heydrich’s order made these councils personally responsible in “the literal sense of the term” for carrying out German orders. When the Nazis sealed the Warsaw Ghetto, the largest of German-occupied Poland’s 400 ghettos, in the fall of 1940, the Jews—then 30 percent of Warsaw’s population—were forced into 2.4 percent of the city’s area. The ghetto’s population reached a density of more than 200,000 persons per square mile (77,000 per square km) and 9.2 per room. Disease, malnutrition, hunger, and poverty took their toll even before the first bullet was fired.
In 1935, Mengele earned a PhD in anthropology from the University of Munich. In January 1937, he joined the Institute for Hereditary Biology and Racial Hygiene in Frankfurt, where he worked for Dr. Otmar Freiherr von Verschuer, a German geneticist with a particular interest in researching twins. As von Verschuer's assistant, Mengele focused on the genetic factors that result in a cleft lip and palate, or a cleft chin. His thesis on the subject earned him a cum laude doctorate in medicine (MD) from the University of Frankfurt in 1938. (Both of his degrees were revoked by the issuing universities in the 1960s.) In a letter of recommendation, von Verschuer praised Mengele's reliability and his ability to verbally present complex material in a clear manner. The American author Robert Jay Lifton notes that Mengele's published works were in keeping with the scientific mainstream of the time, and would probably have been viewed as valid scientific efforts even outside Nazi Germany.