In October 1941, Himmler authorized SS General Odilo Globocnik (SS and police leader for the Lublin District of the Generalgouvernement) to implement a plan to systematically murder all Jews residing in the Generalgouvernement. In 1942, this project received the code name "Operation Reinhard" (Einsatz Reinhard), a reference to Heydrich (who had been authorized to manage the implementation of the "Final Solution" and who was assassinated by Czech agents in May 1942 in Prague).

Mengele became interested in using twins for medical research through his mentor, Verschuer. Verschuer himself was famous for experimenting with identical and fraternal twins in order to trace the genetic origins of various diseases. During the 1930s, twin research was seen as an ideal tool in weighing the factors of human heredity and environment. Mengele and his mentor had performed a number of legitimate research protocols using twins as test subjects throughout the 1930s. Now, at Auschwitz, with full license to maim or kill his subjects, Mengele performed a broad range of agonizing and often lethal experiments with Jewish and Roma (Gypsy) twins, most of them children.
Death: Dr. Miklos Nyiszli was Mengele's prisoner pathologist. The autopsies became the final experiment. Nyiszli performed autopsies on twins who had died from the experiments or who had been purposely killed just for after-death measurements and examination. Some of the twins had been stabbed with a needle that pierced their heart and then was injected with chloroform or phenol, which caused near immediate blood coagulation and death. Some of the organs, eyes, blood samples and tissues would be sent to Verschuer for further study.
Twins were subjected to weekly examinations and measurements of their physical attributes by Mengele or one of his assistants.[49] The experiments he performed on twins included unnecessary amputation of limbs, intentionally infecting one twin with typhus or some other disease, and transfusing the blood of one twin into the other. Many of the victims died while undergoing these procedures,[50] and those who survived the experiments were sometimes killed and their bodies dissected once Mengele had no further use for them.[51] Nyiszli recalled one occasion on which Mengele personally killed fourteen twins in one night by injecting their hearts with chloroform.[31] If one twin died from disease, he would kill the other twin to allow comparative post-mortem reports to be produced for research purposes.[52]
Mengele's eye experiments included attempts to change the eye color by injecting chemicals into the eyes of living subjects, and he killed people with heterochromatic eyes so that the eyes could be removed and sent to Berlin for study.[53] His experiments on dwarfs and people with physical abnormalities included taking physical measurements, drawing blood, extracting healthy teeth, and treatment with unnecessary drugs and X-rays.[3] Many of his victims were dispatched to the gas chambers after about two weeks, and their skeletons sent to Berlin for further analysis.[54] Mengele sought out pregnant women, on whom he would perform experiments before sending them to the gas chambers.[55] Witness Vera Alexander described how he sewed two Romani twins together, back to back, in a crude attempt to create conjoined twins;[50] both children died of gangrene after several days of suffering.[56]

Soon after he became chancellor, Hitler called for new elections in an effort to get full control of the Reichstag, the German parliament, for the Nazis. The Nazis used the government apparatus to terrorize the other parties. They arrested their leaders and banned their political meetings. Then, in the midst of the election campaign, on February 27, 1933, the Reichstag building burned. A Dutchman named Marinus van der Lubbe was arrested for the crime, and he swore he had acted alone. Although many suspected the Nazis were ultimately responsible for the act, the Nazis managed to blame the Communists, thus turning more votes their way.

After the September 1939 German invasion of Poland (the beginning of World War II), anti-Jewish policy escalated to the imprisonment and eventual murder of European Jewry. The Nazis first established ghettos (enclosed areas designed to isolate and control the Jews) in the Generalgouvernement (a territory in central and eastern Poland overseen by a German civilian government) and the Warthegau (an area of western Poland annexed to Germany). Polish and western European Jews were deported to these ghettos where they lived in overcrowded and unsanitary conditions with inadequate food.

Most of the Jewish ghettos of General Government were liquidated in 1942–1943, and their populations shipped to the camps for extermination.[349][350][t] About 42,000 Jews were shot during the Operation Harvest Festival on 3–4 November 1943.[351] At the same time, rail shipments arrived regularly from western and southern Europe at the extermination camps.[352] Few Jews were shipped from the occupied Soviet territories to the camps: the killing of Jews in this zone was mostly left in the hands of the SS, aided by locally recruited auxiliaries.[353][u]

Germany implemented the persecution of the Jews in stages. Following Adolf Hitler's appointment as German Chancellor in January 1933, the regime built a network of concentration camps in Germany for political opponents and those deemed "undesirable", starting with Dachau on 22 March 1933.[5] After the passing of the Enabling Act on 24 March,[6] which gave Hitler plenary powers, the government began isolating Jews from civil society, which included a boycott of Jewish businesses in April 1933 and enacting the Nuremberg Laws in September 1935. On 9–10 November 1938, during Kristallnacht (the "Night of Broken Glass"), Jewish businesses and other buildings were ransacked, smashed or set on fire throughout Germany and Austria, which Germany had annexed in March that year. After Germany invaded Poland in September 1939, triggering World War II, the regime set up ghettos to segregate Jews. Eventually thousands of camps and other detention sites were established across German-occupied Europe.
Following the invasion of the Soviet Union in June 1941, Einsatzgruppen and, later, militarized battalions of Order Police officials, moved behind German lines to carry out mass-murder operations against Jews, Roma, and Soviet state and Communist Party officials. German SS and police units, supported by units of the Wehrmacht and the Waffen SS, murdered more than a million Jewish men, women, and children, and hundreds of thousands of others.

Hilberg, Raul (1985). The Destruction of the European Jews: The Revised and Definitive Edition. New York: Holmes and Meier. ISBN 0-8419-0832-X – via search inside. The deportations ... were the work of a much larger apparatus that had to deal with a host of constraints and requirements. The effort, as we shall see, was deemed necessary to accomplish the Final Solution on a European-wide scale.[p.273] 
Political dissidents, trade unionists, and Social Democrats were among the first to be arrested and incarcerated in concentration camps. Under the Weimar government, centuries-old prohibitions against homosexuality had been overlooked, but this tolerance ended violently when the SA (Storm Troopers) began raiding gay bars in 1933. Homosexual intent became just cause for prosecution. The Nazis arrested German and Austrian male homosexuals—there was no systematic persecution of lesbians—and interned them in concentration camps, where they were forced to wear special yellow armbands and later pink triangles. The goal of persecuting male homosexuals was either for reeducation—what might now be called conversion therapy—or punishment. Jehovah’s Witnesses were a problem for the Nazis because they refused to swear allegiance to the state, register for the draft, or utter the words “Heil Hitler.” As a result, the Nazis imprisoned many of the roughly 20,000 Jehovah’s Witnesses in Germany. They could be released from concentration camps if they signed a document renouncing their faith and promising not to proselytize. Few availed themselves of that option, preferring martyrdom to apostasy. Germans of African descent—many of whom, called “Rhineland bastards” by the Nazis, were the offspring of German mothers and French colonial African troops who had occupied the Rhineland after World War I—were also persecuted by the Nazis. Although their victimization was less systematic, it included forced sterilization and, often, internment in concentration camps. The fear was that they would “further pollute” and thereby diminish the race. The Nazis also singled out the Roma and Sinti, pejoratively known as Gypsies. They were the only other group that the Nazis systematically killed in gas chambers alongside the Jews. For the Roma and Sinti, too, racial pollution and their depiction as asocials was the justification for their persecution and murder.
Dr. Mengele had a Ph.D. in Anthropology as well as a degree in medicine, which he received in July 1938 from the University of Frankfurt. He earned his Ph.D. in 1935 with a thesis on "Racial Morphological Research on the Lower Jaw Section of Four Racial Groups." In January 1937, Dr. Mengele was appointed a research assistant at the Institute for Heredity, Biology and Racial Purity at the University of Frankfurt. He worked under Professor Otmar Freiherr von Verschuer, a geneticist who was doing research on twins. As the war-time director of the Kaiser Wilhelm Institute of Anthropology, Human Hereditary Teaching Genetics, located in Berlin, von Verschuer secured the funds for Mengele's experiments at Auschwitz. The results of Mengele's research on twins was sent to this Institute. The grant for Mengele's genetic research was authorized by the German Research Council in August 1943.
Using gas vans, Chełmno had its roots in the Aktion T4 euthanasia program.[273] Majdanek began as a POW camp, but in August 1942 it had gas chambers installed.[274] A few other camps are occasionally named as extermination camps, but there is no scholarly agreement on the additional camps; commonly mentioned are Mauthausen in Austria[275] and Stutthof.[276] There may also have been plans for camps at Mogilev and Lvov.[277]
He was capable of being so kind to the children, to have them become fond of him, to bring them sugar, to think of small details in their daily lives, and to do things we would genuinely admire ... And then, next to that, ... the crematoria smoke, and these children, tomorrow or in a half-hour, he is going to send them there. Well, that is where the anomaly lay.[48]