Before the start of World War II, around 9.5 million Jewish people lived in Europe. By the time the war ended, the Nazis had killed 6 million European Jews in concentration camps, or pogroms, or ghettos, or mass executions in what we refer to today as the Holocaust. The Nazis used the term Endlösung, or Final Solution, as the “answer” to the “Jewish question.” But when did this monstrous plan get put in motion?
The Nazis considered Jews to be the main danger to Germany. Jews were the primary victims of Nazi racism, but other victims included Roma (Gypsies) and people with mental or physical disabilities. The Nazis murdered some 200,000 Roma. And they murdered at least 250,000 mentally or physically disabled patients, mainly German and living in institutions, in the so-called Euthanasia Program.
Browning believes that the "Final Solution as it is now understood—the systematic attempt to murder every last Jew within the German grasp"[5] took shape during a five-week period, from 18 September to 25 October 1941. During this time: the sites of the first extermination camps were selected, different methods of killing were tested, Jewish emigration from the Third Reich was forbidden, and 11 transports departed for Łódź as a temporary holding station. During this period, Browning writes, "The vision of the Final Solution had crystallised in the minds of the Nazi leadership, and was being turned into reality."[5] This period was the peak of Nazi victories against the Soviet Army on the Eastern Front, and, according to Browning, the stunning series of German victories led to both an expectation that the war would soon be won, and the planning of the final destruction of the "Jewish-Bolshevik enemy".[114]
These programmes are best seen as a series of linked genocides, each having its own history, background, purpose and significance in the Nazi scheme of things. The Holocaust was the biggest of the killing programmes and, in certain important ways, different from the others. The Jews figured in Nazi ideology as the arch-enemy of the 'Aryan race', and were targeted not merely for terror and repression but for complete extinction. The Nazis failed in this aim because they ran out of time, but they pursued it fanatically until their defeat in 1945. The Holocaust led to widespread public awareness of genocide and to modern efforts to prevent it, such as the 1948 UN Convention on Genocide.
Construction work on the first killing centre at Bełżec in occupied Poland began in October 1941, three months before the Wannsee Conference. The new facility was operational by March the following year.[75] By mid-1942, two more death camps had been built on Polish lands: Sobibór operational by May 1942, and Treblinka operational in July.[76] From July 1942, the mass murder of Polish and foreign Jews took place at Treblinka as part of Operation Reinhard, the deadliest phase of the Final Solution. More Jews were killed at Treblinka than at any other Nazi extermination camp apart from Auschwitz.[77] By the time the mass killings of Operation Reinhard ended in 1943, roughly two million Jews in German-occupied Poland had been murdered.[66] The total number of people killed in 1942 in Lublin/Majdanek, Bełżec, Sobibór, and Treblinka was 1,274,166 by Germany's own estimation, not counting Auschwitz II Birkenau nor Kulmhof.[78] Their bodies were buried in mass graves initially.[79] Both Treblinka and Bełżec were equipped with powerful crawler excavators from Polish construction sites in the vicinity, capable of most digging tasks without disrupting surfaces.[80] Although other methods of extermination, such as the cyanic poison Zyklon B, were already being used at other Nazi killing centres such as Auschwitz, the Aktion Reinhard camps used lethal exhaust gases from captured tank engines.[81]
While the Nazis murdered other national and ethnic groups, such as a number of Soviet prisoners of war, Polish intellectuals, and gypsies, only the Jews were marked for systematic and total annihilation. Jews were singled out for "Special Treatment" (Sonderbehandlung), which meant that Jewish men, women and children were to be methodically killed with poisonous gas. In the exacting records kept at the Auschwitz death camp, the cause of death of Jews who had been gassed was indicated by "SB," the first letters of the two words that form the German term for "Special Treatment."
General Patch's 12th Armored Division, forging their way towards the Austrian border, uncovered horrors at a German prison camp at Schwabmunchen, southwest of Munich. Over 4,000 slave laborers, all Jews of various nationalities, were housed in the prison. The internees were burned alive by guards who set fire to the crude huts in which the prisoners slept, shooting any who tried to escape. Sprawled here in the prison enclosure are the burnt bodies of some of the Jewish slave laborers uncovered by the US 7th Army at Schwabmunchen, May 1, 1945. #
The Nazis used the phrase Lebensunwertes Leben (life unworthy of life) in reference to the disabled and mentally ill.[92] On 14 July 1933, the Law for the Prevention of Hereditarily Diseased Offspring (Gesetz zur Verhütung erbkranken Nachwuchses), the Sterilization Law, was passed, allowing for compulsory sterilization.[93][94] The New York Times reported on 21 December that year: "400,000 Germans to be sterilized".[95] There were 84,525 applications from doctors in the first year. The courts reached a decision in 64,499 of those cases; 56,244 were in favor of sterilization.[96] Estimates for the number of involuntary sterilizations during the whole of the Third Reich range from 300,000 to 400,000.[97]

Another Polish courier, Jan Karski, reached the west in November 1942, carrying messages from Jewish leaders in Poland. He had himself witnessed the conditions in the Warsaw ghetto and in what he believed to be the Belzec death camp, and was eager to inform the world. Karski saw the British foreign secretary, Anthony Eden, and US President Roosevelt, but they seemed to be more interested in military intelligence than in atrocity stories. Partly as a result of Karski's mission, however, the Allies agreed to a joint declaration, read to the British Parliament on 17 December, which acknowledged Nazi war crimes and threatened punishment for the perpetrators. Subsequently millions of leaflets were dropped in the course of bombing raids on German cities to inform Germans of the facts, but these had little or no effect.

Half a year later, Harel was replaced by Meir Amit, who ordered the Mossad to “stop chasing after ghosts from the past and devote all our manpower and resources to threats against the security of the state.” He mandated that the agency deal with Nazis “only to the extent it is able to do so, in addition to its principal missions” and as long as “it doesn’t impinge on the other operations.”
Sources: C.A.N.D.L.E.S; Kor, Eva Mozes. Echoes from Auschwitz. IN.: C.A.N.D.L.E.S. 1995; Lifton, Robert Jay. The Nazi Doctors. The United States: Basic Books. 1986; Nyiszli., Dr. Miklos. Auschwitz: A Doctor's Eyewitness Account. New York: Fawcett Crest. 1960; Posner, Gerald L. and John Ware. Mengele: The Complete Story. New York: Dell Publishing. 1986; Ramati, Alexander. And the Violins Stopped Playing
In the years of Nazi rule before World War II, policies of persecution and segregation targeting German Jews focused on the goal of expulsion. After the Nazi party seized power in 1933, state-sponsored racism generated anti-Jewish legislation, boycotts, "Aryanization," and massive street violence, as in the Kristallnacht (commonly known as the "Night of Broken Glass") pogroms. With all of these measures, the Nazi leaders sought to drive the Jews out of Germany by systematically isolating them from German society and by eliminating them from the German economy, removing any opportunity for them to make a living in Germany.
One extraordinary aspect of the journey to the death camps was that the Nazis often charged Jews deported from Western Europe train fare as third class passengers under the guise that they were being "resettled in the East." The SS also made new arrivals in the death camps sign picture postcards showing the fictional location "Waldsee" which were sent to relatives back home with the printed greeting: "We are doing very well here. We have work and we are well treated. We await your arrival."
On January 30, 1933, Adolf Hitler, leader of the National Socialist German Workers (Nazi) Party, was named chancellor of Germany by President Paul von Hindenburg after the Nazi party won a significant percentage of the vote in the elections of 1932. The Nazi Party had taken advantage of the political unrest in Germany to gain an electoral foothold. The Nazis incited clashes with the communists and conducted a vicious propaganda campaign against its political opponents - the weak Weimar government and the Jews whom the Nazis blamed for Germany's ills.

It is not known when Hitler formed the intention of the “final solution of the Jewish question” on the scale of the European continent. The conference in Wannsee on January 20, 1942 considered only the details of the undertaking: the methods for organizing the deportation and ensuring the cooperation of the civilian administration. Overall, the plans called for the murder of 11 million Jews living in Germany, the occupied territory, the states opposed to the Third Reich, and the allied and neutral countries. 

It might be noted that, to a surprising extent, much about Hitler’s precise knowledge of the Holocaust remains unclear. For instance, we do not know if Hitler ever saw photographs or newsreels of the killing process, or, indeed, just how comprehensive and brutally frank were Himmler’s reports to Hitler. We have agendas of face-to-face meetings between Hitler and Himmler, at which the Holocaust was to be discussed, but no memorializations or minutes of such meetings. Most of the senior Nazis who were tried at Nuremberg in 1945-1946 (few of whom had any immediate involvement in the killing of Jews) had apparently never seen photographic evidence of the horrors of the concentration camps until their trials; they appeared to be genuinely shocked when newsreels of Belsen and Buchenwald were shown to the court.

Sources: C.A.N.D.L.E.S; Kor, Eva Mozes. Echoes from Auschwitz. IN.: C.A.N.D.L.E.S. 1995; Lifton, Robert Jay. The Nazi Doctors. The United States: Basic Books. 1986; Nyiszli., Dr. Miklos. Auschwitz: A Doctor's Eyewitness Account. New York: Fawcett Crest. 1960; Posner, Gerald L. and John Ware. Mengele: The Complete Story. New York: Dell Publishing. 1986; Ramati, Alexander. And the Violins Stopped Playing

In 1942, Auschwitz II (Birkenau), originally intended to house slave laborers, began to be used instead as a combined labor camp and extermination camp.[23][24] Prisoners were transported there by rail from all over German-occupied Europe, arriving in daily convoys.[25] By July 1942, SS doctors were conducting "selections" where incoming Jews were segregated, and those considered able to work were admitted into the camp while those deemed unfit for labor were immediately killed in the gas chambers.[26] The arrivals that were selected to die, about three-quarters of the total,[a] included almost all children, women with small children, pregnant women, all the elderly, and all of those who appeared (in a brief and superficial inspection by an SS doctor) to be not completely fit and healthy.[28][29]