At the height of World War I, the Young Turk government of the Ottoman Empire suspected its Armenian population of harbouring sympathy for the Russian enemy. Armenians serving in the Ottoman army were taken into camps and killed, as were Armenian political leaders and intellectuals. The mass of the population were rounded up and marched off to concentration camps, where many of them died of hunger, thirst or disease. In 1920–3, when the Ottoman Empire invaded parts of Armenia, Armenians were subject to further mass killings.
It is estimated that up to 1.5 million out of a population of 2.5 million Armenians died. As genocide is defined as a purposeful extermination of an entire people or race, the mass killing of Armenians has been widely accepted as a genocidal act (whose denial became punishable by law in France). Since the founders of the Turkish Republic participated in the Young Turk movement, this is still a hotly contested issue in Turkey, with most Turks (including the Turkish government and most Turkish academics) denying that the genocide ever happened, underlining instead that this was a relocation move necessitated by the realities of war. The Armenian genocide is commonly referred to as the twentieth-century's first genocide, though that term should be applied to the German attempted extermination of the Herero people in its South-West African colony in 1904.