Who authorized the killings?
Hitler authorized the mass killing of those labelled by the Nazis as "undesirables" in the T-4 Euthanasia Program. Hitler encouraged the killings of the Jews of Eastern Europe by the Einsatzgruppen death squads in a speech in July, 1941, though he almost certainly approved the mass shootings earlier. A mass of evidence suggests that sometime in the fall of 1941, Himmler and Hitler agreed in principle on the complete mass extermination of the Jews of Europe by gassing, with Hitler explicitly ordering the "annihilation of the Jews" in a speech on December 12, 1941). To make for smoother intra-governmental cooperation in the implementation of this "Final Solution" to the "Jewish Question", the Wannsee conference was held near Berlin on January 20, 1942, with the participation of fifteen senior officials, led by Reinhard Heydrich and Adolf Eichmann, the records of which provide the best evidence of the central planning of the Holocaust. Just five weeks later on February 22, Hitler was recorded saying "We shall regain our health only by eliminating the Jews" to his closest associates.

Arguments that no documentation links Hitler to "the Holocaust" ignore the records of his speeches kept by Nazi leaders such as Joseph Goebbels and rely on artificially limiting the Holocaust to exclude what we do have documentation on, such as the T-4 Euthanasia Program and the Kristallnacht pogrom.

Who knew about the killings?

Some claim that the full extent of what was happening in German-controlled areas was not known until after the war. However, numerous rumors and eyewitness accounts from escapees and others gave some indication that Jews were being killed in large numbers. Since the early years of the war the Polish government-in-exile published documents and organised meetings to spread word of the fate of the Jews. By early 1941, the British had received information via an intercepted Chilean memo that Jews were being targeted, and by late 1941 they had intercepted information about a number of large massacres of Jews conducted by German police. In the summer of 1942 a Jewish labor organization (the Bund) got word to London that 700,000 Polish Jews had already died, and the BBC took the story seriously, though the United States State Department did not take the news seriously. By the end of 1942, however, the evidence of the Holocaust had become clear and on December 17, 1942 the Allies issued a statement that the Jews were being transported to Poland and killed. The US State Department was aware of the use and the location of the gas chambers of extermination camps, but refused pleas to bomb them out of operation. On May 12, 1943, Polish government-in-exile and Bund leader Szmul Zygielbojm committed suicide in London to protest the inaction of the world with regard to the Holocaust, stating in part in his suicide letter:

I cannot continue to live and to be silent while the remnants of Polish Jewry, whose representative I am, are being killed. My comrades in the Warsaw ghetto fell with arms in their hands in the last heroic battle. I was not permitted to fall like them, together with them, but I belong with them, to their mass grave.

By my death, I wish to give expression to my most profound protest against the inaction in which the world watches and permits the destruction of the Jewish people.
Debate also continues on how much average Germans knew about the Holocaust. Recent historical work suggests that the majority of Germans knew that Jews were being indiscriminately killed and persecuted, even if they did not know of the specifics of the death camps. Robert Gellately, a historian at Oxford University, conducted a widely-respected survey of the German media before and during the war, concluding that there was "substantial consent and active participation of large numbers of ordinary Germans" in aspects of the Holocaust, and documenting that the sight of columns of slave labourers were common, and that the basics of the concentration camps, if not the extermination camps, were widely known.

Historical interpretations
Why did people participate in, authorize, or tacitly accept the killing?
Obedience
Stanley Milgram was one of a number of post-war psychologists and sociologists who tried to address why people obeyed immoral orders in the Holocaust. Milgram's findings demonstrated that reasonable people, when instructed by a person in a position of authority, obeyed commands entailing what they believed to be the death or suffering of others. These results were confirmed in other experiments as well, such as the Stanford prison experiment.

Functionalism versus intentionalism
A major issue in contemporary Holocaust studies is the question of functionalism versus intentionalism. The terms were coined in a 1981 article by the British Marxist historian Timothy Mason to describe two schools of thought about the origins of the Holocaust. Intentionalists hold that the Holocaust was the result of a long-term masterplan on the part of Hitler's and that Hitler was the driving force behind the Holocaust. Functionalists hold that Hitler was anti-Semitic, but that he did not have a masterplan for genocide. Functionalists see the Holocaust as coming from below in the ranks of the German bureaucracy with little or no involvement on the part of Hitler. Functionalists stress that the Nazi anti-Semitic policy was constantly evolving in ever more radical directions and the end product was the Holocaust.

Intentionalists like Lucy Dawidowicz argue that the Holocaust was planned by Hitler from the very beginning of his political career, at very least from 1919 on, if not earlier. Later Dawidowicz was to date the decision for genocide back to November 11, 1918. Other Intentionalists like Andreas Hillgruber, Karl Dietrich Bracher and Klaus Hildebrand suggested that Hitler had decided upon the Holocaust sometime in the early 1920s. More recent intentionalist historians like Eberhard Jäckel continue to emphasize the relative earliness of the decision to kill the Jews, although they are not willing to claim that Hitler planned the Holocaust from the beginning. Yet another group of intentionalist historians such as the American Arno J. Mayer claimed Hitler only ordered the Holocaust in December 1941.

Functionalists like Hans Mommsen, Martin Broszat, Götz Aly, Raul Hilberg and Christopher Browning hold that the Holocaust was started in 1941-1942 as a result of the failure of the Nazi deportation policy and the impending military losses in Russia. They claim that what some see as extermination fantasies outlined in Hitler's Mein Kampf and other Nazi literature were mere propaganda and did not constitute concrete plans. In Mein Kampf Hitler repeatly states his inexorable hatred of the Jewish people, but no-where does he proclaim his intention to exterminate the Jewish people.

Furthermore, Functionalists point to the fact that in the 1930s, Nazi policy aimed at trying to make life so unpleasant for German Jews that they would leave Germany. Adolf Eichmann was in charge of faciliating Jewish emigration by whatever means possible from 1937 on, until October 3, 1941 were German Jews forbidden to leave, when Reinhard Heydrich issued a order to that effect.

Functionalists point to the SS's support for a time in the late 1930s for Zionist groups as the preferred solution to the "Jewish Question" as another sign that there was no masterplan for genocide. The SS only ceased their support for German Zionist groups in May 1939 when Joachim von Ribbentrop informed Hitler of this, and Hitler ordered Himmler to cease and desist as the creation of Israel was not a goal Hitler thought worthy of German foreign policy.

In particular, Functionalists have noted that in German documents from 1939 to 1941, the term "Final Solution to the Jewish Question" was clearly meant to be a "territorial solution", that is the entire Jewish population was to be expelled somewhere far from Germany and not allowed to come back.

At first, the SS planned to create a gigantic "Jewish Reservation" in the Lublin, Poland area, but the so-called "Lublin Plan" was vetoed by Hans Frank, the Governor-General of Poland who refused to allow the SS to ship any more Jews to the Lublin area after November, 1939. The reason why Frank vetoed the "Lublin Plan" was not due to any humane motives, but rather because he was opposed to the SS "dumping" Jews into the Government-General. In 1940, the SS and the German Foreign Office had the so-called "Madagascar Plan" to deport the entire Jewish population of Europe to a "reservation" on Madagascar. The "Madagascar Plan" was cancelled because Germany could not defeat Britain and until the British blockade was broken, the "Madagascar Plan" could not be put into effect.

Finally, Functionalist historians have made much of a memorandum written by Himmler in May, 1940 explicitly rejecting extermination of the entire Jewish people as "un-German" and going on to recommend to Hitler the "Madagascar Plan" as the preferred "territorial solution" to the "Jewish Question". Not until July 1941 did the term "Final Solution to the Jewish Question" come to mean extermination.

Recently, a synthesis of the two schools has emerged that has been championed by such diverse historians such as the Canadian historian Michael Marrus, the Israeli historian Yehuda Bauer and the British historian Ian Kershaw that contends that Hitler was the driving force behind the Holocaust, but that he did not have a long-term plan and that much of the initiative for the Holocaust came from below in an effort to meet Hitler's perceived wishes.

Another controversy was started by the sociologist Daniel Goldhagen, who argues that ordinary Germans were knowing and willing participants in the Holocaust, which he claims had its roots in a deep eliminationist German anti-Semitism. Most other historians have disagreed with Goldhagen's thesis, arguing that while anti-Semitism undeniably existed in Germany, Goldhagen's idea of a uniquely German "eliminationist" anti-Semitism is untenable, and that the extermination was unknown to many and had to be enforced by the dictatorial Nazi apparatus.

Revisionists and deniers

Holocaust denial, also called Holocaust revisionism, is the belief that the Holocaust did not occur, or, more specifically: that far fewer than around six million Jews were killed by the Nazis (numbers below one million, most often around 300,000 are typically cited); that there never was a centrally-planned Nazi attempt to exterminate the Jews; and/or that there were not mass killings at the extermination camps. Those who hold this position often further claim that Jews and/or Zionists know that the Holocaust never occurred, yet that they are engaged in a massive conspiracy to maintain the illusion of a Holocaust to further their political agenda. These views are not accepted as credible by historians, with organizations such as the American Historical Association, the largest society of historians in the United States, stating that Holocaust denial is "at best, a form of academic fraud."
Some Revisionists have argued that to exterminate 9 to 11 million people in total could not be possible in the short space of time the Nazis were in power between 1941 and 1945. This would mean that over 10000 people would be exterminated by the Nazis each day they were in power.
Holocaust deniers almost always prefer to be called Holocaust revisionists. Most scholars contend that the latter term is misleading. Historical revisionism is a well-accepted and mainstream part of the study of history; it is the reexamination of accepted history, with an eye towards updating it with newly discovered, more accurate, and/or less biased information, or viewing known information from a new perspective. In contrast, Holocaust deniers typically willfully misuse or ignore historical records in order to attempt to prove their conclusions, as Gordon McFee writes:

'Revisionists' depart from the conclusion that the Holocaust did not occur and work backwards through the facts to adapt them to that preordained conclusion. Put another way, they reverse the proper methodology […], thus turning the proper historical method of investigation and analysis on its head.

Public Opinion Quarterly summarized that: "No reputable historian questions the reality of the Holocaust, and those promoting Holocaust denial are overwhelmingly anti-Semites and/or neo-Nazis," though Holocaust denial has also become popular in recent years among Islamic fundamentalists. In late 2005, Iranian president Mahmoud Ahmadinejad denounced the Holocaust of European Jewry as a "myth". Denying the Holocaust is a crime in some countries (including France, Poland, Austria, Switzerland, Belgium, The Netherlands and Germany).

Aftermath

Displaced Persons and the State of Israel

The Holocaust and its aftermath left millions of refugees, including many Jews who had lost most or all of their family members and possessions, and often faced persistent anti-Semitism in their home countries. The original plan of the Allies was to repatriate these "Displaced Persons" to their country of origin, but many refused to return, or were unable to as their homes or communities had been destroyed. As a result, more than 250,000 languished in DP camps for years after the war ended.

Jews were smuggled into Palestine by Berihah using a number of routes.

While Zionism had been prominent before the Holocaust, afterwards it became almost universally accepted among Jews. Many Zionists, pointing to the fact that Jewish refugees from Germany and Nazi-occupied lands had been turned away by other countries, argued that if a Jewish state had existed at the time, the Holocaust could not have occurred on the scale it did. With the rise of Zionism, Palestine became the destination of choice for Jewish refugees, but local Arabs opposed the immigration, Britain refused to allow Jewish refugees into the Mandate, and many countries in the Soviet Bloc made any emigration illegal. Former Jewish partisans in Europe, along with the Haganah in Palestine, organized a massive effort to smuggle Jews into Palestine, called Berihah, which eventually transported 250,000 Jews (both DPs and those who hid during the war) to the Mandate. By 1952, the Displaced Persons camps were closed, with over 80,000 Jewish DPs in the United States, about 136,000 in Israel, and another 20,000 in other nations, including Canada and South Africa.

Legal proceedings against Nazis

Defendants at the Nuremberg Trials - Front row: Göring, Heß, von Ribbentrop, and Keitel. Second row: Dönitz, Raeder, Schirach, Sauckel.
There were a number of legal efforts established to bring Nazis and their collaborators to justice. Some of the higher ranking Nazi officials were tried as part of the Nuremberg Trials, presided over by an Allied court; the first international tribunal of its kind. In total, 5,025 Nazi criminals were convicted between 1945-1949 in the American, British and French zones of Germany. Other trials were conducted in the countries in which the defendants were citizens -- in West Germany and Austria, many Nazis were let off with light sentences, with the claim of "following orders" ruled a mitigating circumstance, and many returned to society soon afterwards. An ongoing effort to pursue Nazis and collaborators resulted, famously, in the trial of Holocaust organizer Adolf Eichmann in Israel in 1961.

Legal action against genocide The Holocaust also galvanized the international community to take action against future genocide, including the Convention on the Prevention and Punishment of the Crime of Genocide in 1948. While international human rights law moved forward quickly in the wake of the Holocaust, international criminal law has been slower to advance; after the Nuremberg trials and the Japanese war crime trials it was over forty years until the next such international criminal procedures, in 1993 in Yugoslavia.

Impact on culture

Holocaust theology
On account of the magnitude of the Holocaust, many theologians have re-examined the classical theological views on God's goodness and actions in the world. Some believers and apostates question whether people can still have any faith after the Holocaust, and some of the theological responses to these questions are explored in Holocaust theology.

Art and literature
German philopsopher Theodor Adorno famously commented that "writing poetry after Auschwitz is barbaric," and the Holocaust has indeed had a profound impact on art and literature, for both Jews and non-Jews. Some of the more famous works are by Holocaust survivors or victims, such as Elie Wiesel, Primo Levi, and Anne Frank, but there is a substantial body of literature and art in many languages. Indeed, Paul Celan wrote his poem Todesfuge as a direct response to Adorno's dictum.

The Holocaust has also been the subject of many films, including Oscar winners Schindler's List and Life is Beautiful. With the aging population of Holocaust survivors, there has been increasing attention in recent years to preserving the memory of the Holocaust. The result has included extensive efforts to document their stories, including the Survivors of the Shoah project, as well as institutions devoted to memorializing and studying the Holocaust, including Yad Vashem in Israel and the US Holocaust Museum.

Holocaust Memorial Days
In a unanimous vote, the United Nations General Assembly voted on November 1, 2005, to designate January 27 as the "International Day of Commemoration in Memory of the Victims of the Holocaust."

January 27, 1945 is the day that the former Nazi concentration and extermination camp of Auschwitz-Birkenau was liberated. Even before the UN vote, January 27 was already observed as Holocaust Memorial Day in the United Kingdom since 2001, as well as other countries, including Sweden, Italy, Germany, Finland, Denmark and Estonia.

Israel observes Yom Hashoah, the "Day of Rememberence of the Holocaust," on the 27th day of the Hebrew month of Nisan, which generally falls in April.

Holocaust Committed by the Nazis.