Reinhard Tristan Eugen Heydrich (; German: [anhat tstan n had] (listen); 7 March 1904 4 June 1942) was a high-ranking German SS and police official during the Nazi era and a principal architect of the Holocaust. He was chief of the Reich Security Main Office (including the Gestapo, Kripo, and SD). He was also Stellvertretender Reichsprotektor (Deputy/Acting Reich-Protector) of Bohemia and Moravia. He served as president of the International Criminal Police Commission (ICPC, later known as Interpol) and chaired the January 1942 Wannsee Conference which formalised plans for the "Final Solution to the Jewish Question"the deportation and genocide of all Jews in German-occupied Europe.

Many historians regard Heydrich as the darkest figure within the Nazi regime; Adolf Hitler described him as "the man with the iron heart". He was the founding head of the Sicherheitsdienst (Security Service, SD), an intelligence organisation charged with seeking out and neutralising resistance to the Nazi Party via arrests, deportations, and murders. He helped organise Kristallnacht, a series of coordinated attacks against Jews throughout Nazi Germany and parts of Austria on 910 November 1938. The attacks were carried out by SA stormtroopers and civilians and presaged the Holocaust. Upon his arrival in Prague, Heydrich sought to eliminate opposition to the Nazi occupation by suppressing Czech culture and deporting and executing members of the Czech resistance. He was directly responsible for the Einsatzgruppen, the special task forces that travelled in the wake of the German armies and murdered more than two million people by mass shooting and gassing, including 1.3 million Jews.

Heydrich was mortally wounded in Prague on 27 May 1942 as a result of Operation Anthropoid. He was ambushed by a team of Czech and Slovak soldiers who had been sent by the Czechoslovak government-in-exile to kill the Reich-Protector; the team was trained by the British Special Operations Executive. Heydrich died from his injuries a week later. Nazi intelligence falsely linked the Czech and Slovak soldiers and resistance partisans to the villages of Lidice and Leky. Both villages were razed; the men and boys age 14 and above were shot, and most of the women and children were deported and murdered in Nazi concentration camps.

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.