Aktion T4 (German, pronounced [aktsion te fi]) was a campaign of mass murder by involuntary euthanasia in Nazi Germany. The term was first used in post-war trials against doctors who had been involved in the killings. The name T4 is an abbreviation of Tiergartenstrae 4, a street address of the Chancellery department set up in early 1940, in the Berlin borough of Tiergarten, which recruited and paid personnel associated with Aktion T4. Certain German physicians were authorised to select patients "deemed incurably sick, after most critical medical examination" and then administer to them a "mercy death" (Gnadentod). In October 1939, Adolf Hitler signed a "euthanasia note", backdated to 1 September 1939, which authorised his physician Karl Brandt and Reichsleiter Philipp Bouhler to begin the killing.

The killings took place from September 1939 until the end of the war in 1945; from 275,000 to 300,000 people were killed in psychiatric hospitals in Germany and Austria, occupied Poland and the Protectorate of Bohemia and Moravia (now the Czech Republic). The number of victims was originally recorded as 70,273 but this number has been increased by the discovery of victims listed in the archives of the former East Germany. About half of those killed were taken from church-run asylums, often with the approval of the Protestant or Catholic authorities of the institutions.The Holy See announced on 2 December 1940 that the policy was contrary to divine law and that "the direct killing of an innocent person because of mental or physical defects is not allowed" but the declaration was not upheld by some Catholic authorities in Germany. In the summer of 1941, protests were led in Germany by the Bishop of Mnster, Clemens von Galen, whose intervention led to "the strongest, most explicit and most widespread protest movement against any policy since the beginning of the Third Reich", according to Richard J. Evans.Several reasons have been suggested for the killings, including eugenics, racial hygiene, and saving money. Physicians in German and Austrian asylums continued many of the practices of Aktion T4 until the defeat of Germany in 1945, in spite of its official cessation in August 1941. The informal continuation of the policy led to 93,521 "beds emptied" by the end of 1941. Technology developed under Aktion T4, particularly the use of lethal gas on large numbers of people, was taken over by the medical division of the Reich Interior Ministry, along with the personnel of Aktion T4, who participated in mass murder of Jewish people. The programme was authorised by Hitler but the killings have since come to be viewed as murders in Germany. The number of people killed was about 200,000 in Germany and Austria, with about 100,000 victims in other European countries.

Adolf Hitler (German: [ˈad.ɔlf ˈhɪt.lɐ] (listen); 20 April 1889 – 30 April 1945) was an Austrian-born German politician who was the dictator of Germany from 1933 until his death in 1945. He rose to power as the leader of the Nazi Party, becoming the chancellor in 1933 and then assuming the title of Führer und Reichskanzler in 1934. During his dictatorship, he initiated World War II in Europe by invading Poland on 1 September 1939. He was closely involved in military operations throughout the war and was central to the perpetration of the Holocaust, the genocide of about six million Jews and millions of other victims.

Hitler was born in Austria-Hungary and was raised near Linz. He lived in Vienna later in the first decade of the 1900s and moved to Germany in 1913. He was decorated during his service in the German Army in World War I. In 1919, he joined the German Workers' Party (DAP), the precursor of the Nazi Party, and was appointed leader of the Nazi Party in 1921. In 1923, he attempted to seize governmental power in a failed coup in Munich and was imprisoned with a sentence of five years. In jail, he dictated the first volume of his autobiography and political manifesto Mein Kampf ("My Struggle"). After his early release in 1924, Hitler gained popular support by attacking the Treaty of Versailles and promoting pan-Germanism, anti-Semitism and anti-communism with charismatic oratory and Nazi propaganda. He frequently denounced international capitalism and communism as part of a Jewish conspiracy.

By November 1932, the Nazi Party held the most seats in the German Reichstag, but did not have a majority. As a result, no party was able to form a majority parliamentary coalition in support of a candidate for chancellor. Former chancellor Franz von Papen and other conservative leaders persuaded President Paul von Hindenburg to appoint Hitler as chancellor on 30 January 1933. Shortly after, the Reichstag passed the Enabling Act of 1933 which began the process of transforming the Weimar Republic into Nazi Germany, a one-party dictatorship based on the totalitarian and autocratic ideology of Nazism. Hitler aimed to eliminate Jews from Germany and establish a New Order to counter what he saw as the injustice of the post-World War I international order dominated by Britain and France. His first six years in power resulted in rapid economic recovery from the Great Depression, the abrogation of restrictions imposed on Germany after World War I, and the annexation of territories inhabited by millions of ethnic Germans, which gave him significant popular support.

Hitler sought Lebensraum (lit. 'living space') for the German people in Eastern Europe, and his aggressive foreign policy is considered the primary cause of World War II in Europe. He directed large-scale rearmament and, on 1 September 1939, invaded Poland, resulting in Britain and France declaring war on Germany. In June 1941, Hitler ordered an invasion of the Soviet Union. By the end of 1941, German forces and the European Axis powers occupied most of Europe and North Africa. These gains were gradually reversed after 1941, and in 1945 the Allied armies defeated the German army. On 29 April 1945, he married his longtime lover, Eva Braun, in the Führerbunker in Berlin. Less than two days later, the couple committed suicide to avoid capture by the Soviet Red Army. Their corpses were burned.

Under Hitler's leadership and racially motivated ideology, the Nazi regime was responsible for the genocide of about six million Jews and millions of other victims whom he and his followers deemed Untermenschen (subhumans) or socially undesirable. Hitler and the Nazi regime were also responsible for the killing of an estimated 19.3 million civilians and prisoners of war. In addition, 28.7 million soldiers and civilians died as a result of military action in the European theatre. The number of civilians killed during World War II was unprecedented in warfare, and the casualties constitute the deadliest conflict in history.

Hitler's actions as Führer of Germany are almost universally regarded as gravely immoral. Prominent historian and biographer Ian Kershaw describes Hitler as "the embodiment of modern political evil", and that "never in history has such ruination—physical and moral—been associated with the name of one man".