Major historians of modern Europe define the Holocaust as the mass murder of European Jews by the Nazis, distinguishing it from other Nazi mass murders i.e. the Romani, Russian POWs, the handicapped Action T4, Ukranians, Polish intellectuals, Serbs, and Communists. The focus on the Jews served as a goal for the Nazis: "For a regime dependent on constant mobilization, the Jew served as the constant mobilizing force."
Over time, it built from persecution to industrialized killing. While Adolf Hitler was clearly the prime motivator, control did not centralize until roughly 1938, in the Schutzstaffel (SS) under Heinrich Himmler. Even then, there were distinct factional struggles for authority within the SS.
With respect to the specific targeting against Jews, there is still argument if the Nazis appealed to a preexisting tendency towards German antisemitism, or if they either created the sentiment or simply went ahead without large-scale public knowledge. Several historians argue that in 1933-1939, antisemitism was a priority only for a minority of the Germans.  Others argue there has been a German tendency that only needed tapping. 
- See also: Nazi racial and biological ideology
The ideological underpinnings of the annihilation of the handicapped, Jews and Romani as well as the mass killings of Slavic populations in German-occupied eastern Europe were based on widely accepted theories of the inequality of races. Daniel Jonah Goldhagen, in the controversial Hitler's Willing Executioners, suggests that there is a long tradition of German antisemitism, something to which the current government is extremely careful to avoid. 
Eugenics, incorporating prescientific ideas of racial purity and purification had already existed long before the Nazis came to power. Other inferior groups, in Nazi ideology, included Slavs and Roma. The Holocaust also extended to ideological opponents including Communists and Jehovah's Witnesses.
Leadership and goals
When an SS man at Auschwitz knocked an icicle out of the hand of a thirsty Primo Levi, Levi dangerously talked back to him, asking warum? (Why?) The guard answered, Harum is kein warum (Here there is no why). — Primo Levi
Hitler was at the heart of the Holocaust. While he often said his antisemitism began in Vienna, there is abundant evidence that while he read antisemitic literature and railed about the Jews, he also had friendly relationships with individual Jews. Shirer and Kershaw differ on how prominent his antisemitism was to his fellow soldiers in the First World War. Kershaw suggests that Hitler subsequently invented an earlier transformation to paranoid antisemitism, which probably took place in 1918-1919, is that Hitler was establishing himself, in Mein Kampf and rhetoric of the 1820s, as one who struggled against adversity, and came "above all from his own bitter experiences — to unique insights about society and politics that enabled him without assistance to formulate at the age of abut twenty a rounded 'world-view'." He unquestionably had formative experiences in Vienna, but he later presented many as part of a self-created myth.
He constantly complained, while in the trenches, of the "invisible foes", the Jews and Marxists. It appears to have turned to a commitment to eliminate the Jews either near the end of the war, in 1916 and 1917, recovering from combat wounds in Munich, he observed "I thought I could no longer recognize the city", and became furious about the "Hebrew corruptors of the people".  it was further refined as he met with antisemites in the early 1920s.
Was there an order?
Some recent historians, however, argue that Hitler may have not made an explicit decision and order, but merely expressed desires.
Hitler certainly set the framework, and had there been no Hitler, there would have been no Holocaust. The earliest documentary evidence of Hitler in a central role, and Himmler being aware of a proposal is in an 18 December 1941 appointment book entry, declassified in 1999 by the Russians. His notes say that he met with Hitler and discussed the answer to "the Jewish question" was to "exterminate them as partisans." Bauer reminds us that Hitler tended not to make formal decisions, but to express his wishes. He continues by calling it a result of a "stage by stage development in 1941," and, with respect to the two schools, explains the functionalist view as assuming that central ideology and decisions were less important than had previously been thought, but they agree that "without approval by Hitler and his inner circle, the murder would have been impossible."  He wrote that the Holocaust specifically was a matter of ideology, and intentionalism versus functionalism were outdated ideas.
While there is no confirmation, it had been widely accepted that he gave the orders for the Final Solution verbally to Himmler and Hermann Goering, Goering then being his deputy and economic chief. Himmler then gave orders to Reinhard Heydrich, who was doing detailed planning from 1939 onwards, culminating in the 1942 Wannsee Conference.
The functionalists, also caused structuralists, however, suggest it was more the result of defeats in the East. 
As an indication of the number of agencies involved, consider the range of attendees at the Wannsee Conference, and remember these were the core organizations trusted with the inner secrets.
|SS-Obergruppenführer Reinhard Heydrich||Chief of the RSHA and Reichsprotektor of Bohemia-Moravia, presiding||Assassinated in Prague in June 1942.|
|Dr. Josef Buehler||Generalgouvernment||Tried in Poland for war crimes and executed in Krakow in July 1948|
|Dr. Roland Freisler||Reich Ministry of Justice||Killed in an air-raid in Berlin in February 1945.|
|SS-Gruppenführer Otto Hofmann||Race and Resettlement Main Office (RuSHA)||sentenced to 25 years in prison for war crimes, but was pardoned in 1954. He died in December 1982.|
|SA-Oberführer Gerhard Klopfer||Chancellery of the Nazi Party||charged with war crimes but was released for lack of evidence. He died in January 1987.|
|Ministerialdirektor Friedrich Wilhelm Kritzinger||Chancellery of the Reich||acquitted of war crimes and died in October 1947.|
|SS-Sturmbannführer Dr. Rudolf Lange||Commander of the SD for Latvia||Killed in action in Poland in February 1945|
|Reichsamtleiter Dr. Georg Leibbrandt||Reich Ministry for the Occupied Eastern Territories||charged with war crimes but the case against him was dismissed in 1950. He died in June 1982.|
|Dr. Martin Luther||Reich Foreign Office||finished the war in a concentration camp after falling out with Foreign Minister Joachim von Ribbentrop; died in Berlin in May 1945|
|Gauleiter Dr. Alfred Meyer||Reich Ministry for the Occupied Eastern territories||Suicide, April 1945|
|SS-Gruppenführer Heinrich Müller||Chief of Amt IV (Gestapo), Reich Security Main Office (RSHA)||Most senior Nazi whose fate is unknown|
|Dr. Erich Neumann||Director, Office of the Four Year Plan||Briefly imprisoned and died in mid 1948.|
|SS-Oberführer Dr. Karl Eberhard Schöngarth||SD, assigned to theGeneralgouvernment)||Executed for war crimes (killing British prisoners of war) in May 1946.|
|Dr. Wilhelm Stuckart||Reich Ministry for the Interior||imprisoned for four years before being released for lack of evidence in 1949. He was killed in a car accident in November 1953.|
|SS-Obersturmbannführer Adolf Eichmann||Head of Referat IV B4 (Jewish affairs)of the Gestapo), minutes secretary||executed in Israel in May 1962.|
Himmler did direct Heydrich to begin detailed planning. Even with the Himmler-Heydrich communications, it is insufficient to focus only on the SS role. The ministerial bureaucracy and military, for whom labor was often more important than killing, must be considered.
There were two major factions in the SS, with different goals. The WVHA economic administration, under Oswald Pohl, sought to maximize the gain from the camps, as with slave labor. Its Inspector of Concentration Camps actually ran them, but its Amtsgruppe W for "SS businesses" had the greatest number of defendants in the Pohl Case (NMT)
The other faction was the RSHA security administration, first under Reinhard Heydrich and then Ernst Kaltenbrunner. The RSHA's predecessor, under Heydrich, directed the field killing of the Einsatzgruppen. RSHA offices included the Gestapo, whose Amt IVB, under Adolf Eichmann was the senior SS bureaucrat in charge of handling deportation and transportation to the camps.
In late 1941, General Georg Thomas, head of the War Economy and Armaments office, sent plans for economic exploitation of the conquered territories to Goering, who sent it to Hitler on 26 February 1942. Thomas, with Hitler's consent, was made Goering's coordinator for economic policy on Soviet territory. 
Stage one: Persecution before Power
The early Nazis, before coming to power in 1933, focused on Jews as the "other" and existential enemy. Sturmabteilung (SA) "brownshirts" routinely harassed and attacked Jews, Communists, and others in the streets. Goebbels organized the first Nazi-sponsored pogrom on 13 October 1923. 
Stage two: Emigration and sterilization
The second stage of Nazi policy concerning Jews, from 1933 to 1938, involved the bureaucratic system, both removing Jews from public office, and creating "Jewish desks" (Judenreferat) in most agencies and branches of the government. This established a momentum for discriminatory measures. At this point, however, the basic Nazi policy against the Jews, however, was to encourage voluntary emigration.
During the Berlin Olympics year of 1936, overt antisemitism was reduced, to show the best possible image.
Organizing the camps
The Nazis opened Dachau in 1933, but for political opponents in general, not specifically Jews. Others followed. Himmler put Theodor Eicke in charge of reorganizing the camps, and, in 1934, he created the Totenkopf SS and became Inspector of Concentration Camps.
About 1,000 Jews were murdered in concentration camps inside Germany before 1939; these were distinct from the killing camps that were opened in 1942 in Poland.
Stage three: Forced relocation from Germany
Stage three, from 1938 to September 1939, involved increasingly severe and humiliating restrictions for Jews, with voluntary emigration as a goal.
The most dramatic episode was the pogrom of 9-10 November 1938, known as Kristallnacht, in which Nazis (especially SA men) burned several hundred Jewish synagogues and looted about 8,000 Jewish-owned stores across the country, killing about 100 Jews and injuring thousands. The pogrom is partially explained by the complementary goals of three participants: Joseph Goebbels, who determined the timing; Heinrich Himmler and his SS, which ordered the temporary arrest of 30,000 prominent Jews; and Hermann Goering, who along with several ministries, implemented preexisting plans to exclude Jews from the German economy and confiscate their property. Hitler's role was to approve of these actions.
Expulsion from Germany
By threatening to expel European Jews, Hitler hoped to pressure the international community to increase Jewish immigration quotas quickly and to accept the Reich's monetary demands for loans in order to finance the rearmament of the German defense forces. Hitler used inflammatory speeches to inspire radical German elements to transform the threatening rhetoric into systematic annihilation.
World reaction was overwhelmingly negative; American leaders started to consider war.  Before the war started, Hitler on January 30, 1939 spoke to the Reichstag (parliament), outlining his plan to eliminate the Jewish population under Nazi domination.
Stage four: Relocation outside Germany
Planning for more drastic action had begun, although it was focused on mass emigration or relocation (e.g., the Madagascar Plan proposed by Franz Rademacher of the Reich Foreign Office) of Jews, rather than physical destruction. As part of the German expansion to the East, there were also plans to do away with parts of the Polish population, the "Tannenberg Campaign".
Goering wrote to Heydrich on 31 July 1941
In completion of the task that was entrusted to you in the edict dated January 24, 1939, I herewith charge you with making all the necessary preparations by means of emigration or evacuation in the most convenient way possible, given the present conditions. I herewith charge you with making all the necessary preparations with regard to the organizational, practical and financial aspects for an Overall Solution (Gesamtloesung) of the Jewish question in the German sphere of influence in Europe. Insofar as the competencies of other central organizations are affected, they are to cooperate with you. I further charge you with submitting to me promptly an overall plan of the preliminary organizational, practical and financial measures for the execution of the intended Final Solution (Endloesung) of the Jewish question.
Austria and Czechoslovakia
Policy for the East was still developing at the time of the invasion of Poland. On 29 September 1939, Alfred Rosenberg said Hitler had refined his views after several weeks of experience with Polish Jews. "He wanted to split the territory into three strips:
- Between the Vistula and the Bug; this would be for the whole of Jewry (including the Reich) as well as all other unreliable elements. Build an insuperable wall on the Vistula — even stronger than the one in the West [the Siegfried Line facing France].
- Create a broad cordon along the previous frontier to be Germanized and colonized. This would be a major task for the nation; to create a German granary, a strong peasantry to resettle good Germans from all over the world.
- In between, a form of Polish state. The future would show whether after a few decades the cordon of settlement would have to be pushed farther forward."
These three strips, however, only covered half of Poland, since Poland had been split with the Soviets, in a secret addendum to the Molotov-Ribbentrop Pact, signed on 27 September.
Gen. Johannes Blaskowitz had complained of atrocities as early as 4 September; he was later relieved as military governor. Himmler put Heydrich in charge of Tannenberg, plans for which he shared with the head of the Order Police (ORPO), Kurt Daluege, whom he told that Hitler had given him an "extraordinarily radical...order for the liquidation of various circles of the Polish leadership, [killings] that ran into the thousands. Wilhelm Keitel, on the military side, had confirmed the policy to intelligence chief Wilhelm Canaris on 12 September, "The matter [of the executions of Polish elites] had already been decided by the Fuehrer; the commander of the Army had been informed that if the Wehrmacht refused to be involved, it had to accept the pressure of the SS and the Gestapo. Therefore, in each military district, civilian commanders would be appointed who would carry the responsibility for ethnic extermination."
Heydrich referred to the organized Einsatzgruppen, but there were incidents involving the military. Indeed, on 15 September, he asked the OKW to order the military police to do the shootings themselves, because they were sending too many Poles to the Einsatzgruppen. 
Stage five: Systematic field killing
Stage five began when the Germans invaded Russia in June 1941. Einsatzgruppen of the SS not only massacred large numbers of Jews, but routinely included handicapped persons in open-air mass shootings.
Stage six: the Final Solution
Stage 6 began at the Wannsee Conference in January 1942 began stage five of Nazi power; it was then that when top Nazis decided on a “Final Solution” —to round up and secretly execute all the Jews of Europe. Killing centers were opened in Poland, and thousands of trainloads of Jews were transported there. and Jews were gassed immediately upon arrival.Over three million Jews (and numbers of gypsies and other hated groups) were murdered, mostly in 1942–43.
Stage seven: Endgame
Stage 7 arrived when the Soviet armies overran the camps in Poland in 1944–45 and liberated the survivors. A number of Jews, however, died in death marches as they were evacuated from camps.
Stopping the Holocaust
The historian Yehuda Bauer asks the question "could the Jews murdered in the Holocaust been rescued?" Certainly, he believes, if Germany could have been stopped when it was still weak — but before anyone knew there would be a Holocaust. But if the Jews were allowed to emigrate, where would they go? British foreign policy did not want mass immigration into Palestine, in part because they worried it would destabilize Indian Muslims.  It was fairly unlikely that internal resistance was plausible, even if weapons were available. Some inferences may be drawn that the Treblinka revolt took place only when two complementary leaders worked together: a former Czech army officer with the military skills, Zhelo Bloch, and community leaders including Marceli Galewski. They were also encouraged by their awareness of the Warsaw Ghetto Revolt. 
National government response
In rounding up Jews the Nazis sometimes had the enthusiastic cooperation of pro-Nazi governments (as in France and Slovakia). A few countries, including Italy and Hungary, tried to stall the Nazis, but the Germans took power directly and seized the Jews. Only Bulgaria and Denmark were largely successful in protecting their Jews.
The role of the Vatican remains extremely complex.
Resistance took many forms, from individual acts to hundreds of examples of organized, armed resistance. The most famous episode was the month-long uprising of 60,000 remaining Jews in the Warsaw Ghetto in April 1943.
There were a number of revolts in concentration camps. At Treblinka, resistance began with the killing of an individual SS man, Max Bielas, in 1942. 160 Jews were killed in reprisal.  A much more major revolt, however took place in August 1943, more concerned with disabling the facility than with escape.In Sobibor Concentration Camp, an uprising in October 1943 allowed 600 prisoners to escape.
The Allies liberated the concentration camps in 1945. Questions have been asked if they could have done more to intervene.
First, the Allied high command did not necessarily realize the extent of the Holocaust or its facilities. British communications intelligence intercepted German Police messages, between July and September 1941, indicated that large-scale killing, especially of Jews, was in progress. At least one report reached Churchill. The Germans imposed greater communications security and this source dried up. Remember that the Germans themselves had not decided yet on the Final Solution. Churchill referred to it in a propaganda speech in August, but there was no further use. The British were also concerned about revealing their decoding capability.
Some information was available in raw form, but not usable. There were photographs of the Auschwitz Concentration Camp, they were not located and interpreted until 1970. A photographic plane was photographing a I.G. Farben factory in the general area, and didn't turn off its camera until after it had passed over the Monowitz camp . The factory was the main interest, and the WWII interpreters just marked Auschwitz as an unidentified installation. No one in that organization knew about human intelligence reports of the death camps, and only in the seventies did researchers learn the significance of the camp photographs. .
Brugioni explains why Allied intelligence knew little about the targets, even after the President asked that the camps be bombed. "When the bombing specialists were ordered to formulate plans for bombing the Auschwitz-Birkenau Extermination Complex, officials of the Air Ministry, the Royal Air Force Bomber Command and the U.S. 8th Air Force bemoaned the lack of aerial photographic coverage of the complex. In fact, such photos were readily available at the Allied Central Interpretation Unit at Royal Air Force Station Medmenham, 50 miles outside of London and at the Mediterranean Allied Photo Reconnaissance Wing in Italy. The ultimate irony was that no search for the aerial photos was ever instituted by either organization. In retrospect, it is a fact that by the time the Soviet Army reached Auschwitz on January 27, 1945, the Allies had photographed the Auschwitz-Birkenau Extermination Complex at least 30 times."
Refugee and underground human-source intelligence reports were not regarded as highly credible.
Diversion from defeating Germany
Second, there was a belief that defeating Germany in general was the best way to stop the killing. In 1943, extermination camps in Poland were beyond the range of serious bombing missions from the Western Allies, unless the aircraft could refuel in Russia. Refueling rights were denied.
If bombers had been able to reach the camps, they probably would have been most effective if they destroyed the gas chambers and crematoria, but 1943-1944 bombing accuracy was not a major threat to specific buildings. While it ws requested the railroads be bombed, railroads are fairly easily repaired, and require a continued campaign to keep out of action.
Lack of sympathy
Third, there may have been some degree of antisemitism that prevented it from being a high priority. The Soviets were closer to the extermination camps, but antisemitism was Soviet policy.
The main war criminals were tried at the Nuremberg Trials (International Military Tribunal of the Major War Criminals, IMT) in 1945–1947, and at smaller trials throughout Europe, especially the U.S. Nuremberg Military Tribunals. Individuals were also tried in the countries where atrocities were committed, such as Rudolf Hoess, the first commandant of Auschwitz Concentration Camp, in Poland, following his testimony at the IMT.
- Snyder, Timothy (2010). Bloodlands : Europe between Hitler and Stalin. New York: Basic Books. ISBN 0-465-00239-0. p.389. Gilbert, Martin (2000). Never again : a history of the Holocaust. New York, NY: Universe. ISBN 0-7893-0409-0. p.18.
- Saul Friedlander (2007), The Years of Extermination: Nazi Germany and the Jews, 1939-1945, HarperCollins, p. xix
- Christopher R. Browning (2004), The Origins of the Final Solution: The evolution of Nazi Jewish policy, September 1939-March 1942, University of Nebraska Press and Yad Vashem, ISBN 0803213271, p. 9
- Daniel Jonah Goldhagen (1997), Hitler's Willing Executioners: Ordinary Germans and the Holocaust, Vintage, p. 9
- Goldhagen, p. 5
- Jerry Frank (2006), Drang nach Osten: The German Migration to the East
- Ian Kershaw (1998), Hitler 1889-1936: Hubris, W.W. Norton, ISBN 0-393-04671-0, pp. 64-65
- William Shirer (1960), The Rise and Fall of the Third Reich, Simon & Schuster, pp. 30-31
- Joachim Fest (1973), Hitler, Harcourt Brace Jovanovich, pp. 74-75
- Yehuda Bauer (2001), Rethinking the Holocaust, Yale University Press, ISBN 030008222568, p. 5
- Yehuda Bauer (2002), Rethinking the Holocaust, Yale University Press, p. 176
- Bauer, p. 4-7
- Lucy Dawidowicz (1975), The War against the Jews, 1933-1945 (10th Anniversary ed.), Bantam Books, ISBN 0-553-34532-X, pp. 6-7 Cite error: Invalid
<ref>tag; name "WATJ" defined multiple times with different content
- Dawidowicz, p. xxix,
- Browning, p. 214
- Joachim C. Fest (1973), Hitler, Harcourt Brace Jovanovich, p. 303
- Ian Kershaw (2000), Hitler 1936-45: Nemesis, W.W. Norton, ISBN 0393049949, p. 5
- "Hitler's Reichstag Speech of 30 January 1939." 
- Stefan Kley (2000), "Hitler and the Pogrom of November 9-10, 1938", Yad Vashem Studies
- Friedlander 2007, pp. 238-239
- Friedlander 2007, p. 11
- Johannes Blaskowitz, Jewish Virtual Library
- Friedlander 2007, p. 13
- Czeslaw Madachczyk, Die Okkupationpolitik Nazideutschlands in Polen 1939-1945, p. 28, quoted in Browning 2004, p. 29
- Goldhagen, pp. 355-371
- Yehuda Bauer (2001), Rethinking the Holocaust, Yale University Press, ISBN 030008222568, p. 213-214
- Treblinka Death Camp Revolt, United States Holocaust Memorial Museum
- Acts of Resistance and the Organization of the Revolt in Treblinka (1 of 2), Nizkor Project
- Organization of the Underground in the Extermination Area (2 of 2), Nizkor Project
- Thomas Toivi Blatt, Sobibor: the forgotten revolt
- Bauer, pp. 213-222
- Aerial Photographs of Auschwitz. The Auschwitz Album. Yad Vashem: The Holocaust Martyrs' and Heroes' Remembrance Authority (2004). Retrieved on 2007-09-16.
- Brugioni, Dino (Jan-Mar. 1983). "Auschwitz and Birkenau: Why the World War II Photo Interpreters Failed to Identify the Extermination Complex". Military Intelligence 9 (1): 50-55.
- Bauer, pp. 216-218