The Nuremberg trials were held by the Allies against representatives of the defeated Nazi Germany, for plotting and carrying out invasions of other countries, and other crimes, in World War II.

Between 1939 and 1945, Nazi Germany invaded many countries across Europe, inflicting 27 million deaths in the Soviet Union alone. Proposals for how to punish the defeated Nazi leaders ranged from a show trial (the Soviet Union) to summary executions (the United Kingdom). In mid-1945, France, the Soviet Union, the United Kingdom, and the United States agreed to convene a joint tribunal in Nuremberg, with the Nuremberg Charter as its legal instrument. Between 20 November 1945 and 1 October 1946, the International Military Tribunal (IMT) tried 21 of the most important surviving leaders of Nazi Germany in the political, military, and economic spheres, as well as six German organizations. The purpose of the trial was not just to convict the defendants but also to assemble irrefutable evidence of Nazi crimes, offer a history lesson to the defeated Germans, and delegitimize the traditional German elite.

The IMT focused on the crime of aggressionplotting and waging aggressive war, which the verdict declared "the supreme international crime" because "it contains within itself the accumulated evil of the whole". This crime had been invented by Soviet jurist Aron Trainin during the war. Most of the defendants were also charged with war crimes and crimes against humanity. Twelve further trials were conducted by the United States against lower-level perpetrators, which focused more on the Holocaust. Although controversial at the time for their use of ex post facto law, the trials' innovation of holding individuals responsible for violations of international law established international criminal law.

Nazism ( NA(H)T-see-iz-əm), the common name in English for National Socialism (German: Nationalsozialismus, German: [natsi̯oˈnaːlzotsi̯aˌlɪsmʊs] (listen)), is the ideology and practices associated with Adolf Hitler and the Nazi Party (German: Nationalsozialistische Deutsche Arbeiterpartei, NSDAP; or National Socialist German Workers' Party in English) in Nazi Germany. During Hitler's rise to power in 1930s Europe, it was frequently referred to as Hitlerism. The later related term "neo-Nazism" is applied to other far-right groups with similar ideas which formed after the collapse of the Nazi regime.

Nazism is a form of fascism, with disdain for liberal democracy and the parliamentary system. It incorporates fervent antisemitism, anti-communism, scientific racism, and the use of eugenics into its creed. Its extreme nationalism originated in pan-Germanism and the ethno-nationalist neopagan Völkisch movement which had been a prominent aspect of German nationalism since the late 19th century, and it was strongly influenced by the Freikorps paramilitary groups that emerged after Germany's defeat in World War I, from which came the party's underlying "cult of violence". Nazism subscribed to pseudo-scientific theories of a racial hierarchy and social Darwinism, identifying the Germans as a part of what the Nazis regarded as an Aryan or Nordic master race. It aimed to overcome social divisions and create a homogeneous German society based on racial purity which represented a people's community (Volksgemeinschaft). The Nazis aimed to unite all Germans living in historically German territory, as well as gain additional lands for German expansion under the doctrine of Lebensraum and exclude those whom they deemed either Community Aliens or "inferior" races.

The term "National Socialism" arose out of attempts to create a nationalist redefinition of socialism, as an alternative to both Marxist international socialism and free-market capitalism. Nazism rejected the Marxist concepts of class conflict and universal equality, opposed cosmopolitan internationalism, and sought to convince all parts of the new German society to subordinate their personal interests to the "common good", accepting political interests as the main priority of economic organisation, which tended to match the general outlook of collectivism or communitarianism rather than economic socialism. The Nazi Party's precursor, the pan-German nationalist and antisemitic German Workers' Party (DAP), was founded on 5 January 1919. By the early 1920s, the party was renamed the National Socialist German Workers' Party to attract workers away from left-wing parties such as the Social Democrats (SPD) and the Communists (KPD), and Adolf Hitler assumed control of the organisation. The National Socialist Program, or "25 Points", was adopted in 1920 and called for a united Greater Germany that would deny citizenship to Jews or those of Jewish descent, while also supporting land reform and the nationalisation of some industries. In Mein Kampf, literally "My Struggle", published in 1925–1926, Hitler outlined the antisemitism and anti-communism at the heart of his political philosophy as well as his disdain for representative democracy and his belief in Germany's right to territorial expansion.The Nazi Party won the greatest share of the popular vote in the two Reichstag general elections of 1932, making them the largest party in the legislature by far, albeit still short of an outright majority. Because none of the parties were willing or able to put together a coalition government, Hitler was appointed Chancellor of Germany in 1933 by President Paul von Hindenburg through the support and connivance of traditional conservative nationalists who believed that they could control him and his party. With the use of emergency presidential decrees by Hindenburg and a change in the Weimar Constitution which allowed the Cabinet to rule by direct decree, bypassing both Hindenburg and the Reichstag, the Nazis soon established a one-party state.

The Sturmabteilung (SA) and the Schutzstaffel (SS) functioned as the paramilitary organisations of the Nazi Party. Using the SS for the task, Hitler purged the party's more socially and economically radical factions in the mid-1934 Night of the Long Knives, including the leadership of the SA. After the death of President Hindenburg, political power was concentrated in Hitler's hands and he became Germany's head of state as well as the head of the government, with the title of Führer, meaning "leader". From that point, Hitler was effectively the dictator of Nazi Germany – also known as the Third Reich – under which Jews, political opponents and other "undesirable" elements were marginalised, imprisoned or murdered. During World War II, many millions of people—including around two-thirds of the Jewish population of Europe—were eventually exterminated in a genocide which became known as the Holocaust. Following Germany's defeat in World War II and the discovery of the full extent of the Holocaust, Nazi ideology became universally disgraced. It is widely regarded as immoral and evil, with only a few fringe racist groups, usually referred to as neo-Nazis, describing themselves as followers of National Socialism.