This level and the next look at five cases that shaped how the world thinks about genocide. They are described plainly and without graphic detail. Numbers are estimates; where scholars differ, ranges are given.

The Armenian genocide (1915 onward)

Armenians were a Christian minority in the Ottoman Empire, where they had long faced discrimination and, in the 1890s, large massacres. By 1913 the empire was controlled by the Committee of Union and Progress, often called the Young Turks, whose leaders increasingly saw the empire's Christian minorities as a threat. The empire entered the First World War in late 1914, and after early military defeats Armenians were accused of disloyalty.

On 24 April 1915, authorities in Constantinople (now Istanbul) arrested hundreds of Armenian community leaders and intellectuals; most were later killed, and the date is now the main day of remembrance. Over the following months Armenians across Anatolia were deported. Many men were killed outright, and columns of women, children and elderly people were marched toward the Syrian desert, where great numbers died from killing, starvation and disease. Estimates of the dead range from about 600,000 to 1.5 million; many scholars put the figure at about one million or more. Assyrian and Greek Christians also suffered mass violence in the same period.

Lemkin cited the Armenians as a reason the law needed a new word. Most genocide scholars and many governments recognize these events as genocide. The government of Turkey, the Ottoman Empire's successor state, acknowledges that many Armenians died but rejects the term genocide, attributing the deaths to wartime conditions. That denial is rejected by the great majority of historians who study the period.

The Holocaust (1933 to 1945)

The Holocaust, also called the Shoah (Hebrew for catastrophe), was the systematic, state-sponsored persecution and murder of about six million Jews by Nazi Germany and its allies and collaborators. The Nazi Party came to power in 1933 with an ideology of racial hierarchy that cast Jews as the nation's mortal enemy.

Persecution moved step by step. Jews were pushed out of jobs and public life. The Nuremberg Laws of 1935 stripped Jews of citizenship and banned marriages with non-Jews. In November 1938 a nationwide pogrom, later called Kristallnacht, destroyed synagogues and Jewish shops. After war began in 1939, Jews in occupied Poland were forced into ghettos. Following the invasion of the Soviet Union in June 1941, mobile killing units carried out mass shootings across the occupied east. From late 1941 the regime built killing centers, including Auschwitz-Birkenau, Treblinka, Sobibor and Belzec, and deported Jews to them by train from across Europe. In January 1942, at the Wannsee Conference, senior officials met to coordinate what they called the “Final Solution”.

The regime also murdered hundreds of thousands of Roma and Sinti, people with disabilities, and millions of Soviet prisoners of war, Poles and others. The Holocaust is unusually well documented by German records, the testimony of survivors and liberators, and postwar trials. Holocaust denial nonetheless exists; historians reject it as contrary to an overwhelming body of evidence.

What the two cases share

• Both took place in wartime and targeted a minority cast as an enemy within.
• Both followed long periods of discrimination and used the machinery of a modern state.
• Both were driven by ideologies that claimed the nation could only be safe or pure without the targeted group.
• Both have been denied, and their memory remains a living issue for descendants and for the states involved.

Worked example: how historians know

Deniers often claim that the evidence for genocide is fabricated or exaggerated. Looking at the kinds of evidence historians use shows why their conclusions are so firm.

For the Armenian genocide, sources include reports from foreign diplomats in the Ottoman Empire, among them the American ambassador Henry Morgenthau and diplomats of Germany, which was the Ottomans' wartime ally and had no reason to invent accusations. Missionaries, aid workers and survivors left detailed accounts. In 1919 and 1920, Ottoman military courts themselves tried several leaders of the ruling party for the deportations and massacres, sentencing some to death in their absence.

For the Holocaust, there are German government documents, railway timetables, and the minutes of the Wannsee Conference, a copy of which was found after the war. There is the testimony of tens of thousands of survivors, of liberating soldiers, and of perpetrators at trials. There are the physical sites, census figures showing the disappearance of whole communities, and photographs taken by Allied forces at liberation.

The key point is convergence. These sources were produced by different people, with different interests, in different languages, often without knowing of one another, and they tell the same story. A denier must explain away every one of them at once. Historians do debate details, such as exact numbers or the timing of particular decisions. Those debates take place within the evidence; denial takes place against it.