Thus although the Nazi 'Final Solution' was one genocide among many, it had features that made it stand out from all the rest as well. Unlike all the others it was bounded neither by space nor by time. It was launched not against a local or regional obstacle, but at a world-enemy seen as operating on a global scale. It was bound to an even larger plan of racial reordering and reconstruction involving further genocidal killing on an almost unimaginable scale, aimed, however, at clearing the way in a particular region – Eastern Europe – for a further struggle against the Jews and those the Nazis regarded as their puppets. It was set in motion by ideologues who saw world history in racial terms. It was, in part, carried out by industrial methods. These things all make it unique.

In the first phase of the experiments, pairs of twins and persons with inherited anomalies were put at the disposal of Dr. Mengele and subjected to all imaginable specialist medical examinations. They were also photographed, plaster casts were made of their jaws and teeth, and they were toe- and fingerprinted. As soon as these examinations were finished, they were killed with lethal injections of phenol to the heart so that the next phase of the experimentation could begin: autopsies and the comparative analysis of their internal organs.
Nazi persecution, arrests, and deportations were directed against all members of Jewish families, as well as many Gypsy families, without concern for age. Homeless, often orphaned, many children had frequently witnessed the murder of parents, siblings, and relatives. They faced starvation, illness, brutal labor, and other indignities until they were consigned to the gas chambers.

Yugoslavia and Greece were invaded in April 1941 and surrendered before the end of the month. Germany and Italy divided Greece into occupation zones but did not eliminate it as a country. Yugoslavia, home to around 80,000 Jews, was dismembered; regions in the north were annexed by Germany and regions along the coast made part of Italy. The rest of the country was divided into the Independent State of Croatia, nominally an ally of Germany, and Serbia, which was governed by a combination of military and police administrators.[167] According to historian Jeremy Black, Serbia was declared free of Jews in August 1942.[168] Croatia's ruling party, the Ustashe, killed the country's Jews, and killed or expelled Orthodox Christian Serbs and Muslims.[167] Jews and Serbs alike were "hacked to death and burned in barns", according to Black. One difference between the Germans and Croatians was that the Ustashe allowed its Jewish and Serbian victims to convert to Catholicism so they could escape death.[168]
In 1935, Mengele earned a PhD in anthropology from the University of Munich.[7] 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.[7] As von Verschuer's assistant, Mengele focused on the genetic factors that result in a cleft lip and palate, or a cleft chin.[10] His thesis on the subject earned him a cum laude doctorate in medicine (MD) from the University of Frankfurt in 1938.[11] (Both of his degrees were revoked by the issuing universities in the 1960s.)[12] In a letter of recommendation, von Verschuer praised Mengele's reliability and his ability to verbally present complex material in a clear manner.[13] 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.[13]