Genocide haunts the 20th century. The Holocaust, the German attempt to exterminate the Jews, comes immediately to mind -- though similar policies against the Roma, gays, and others are less well known. The genocide in Rwanda, where the world watched the massacre of the Tutsi by the Hutu and did nothing, and the Srebrenica massacre of Bosnian Muslims by Serbs are fresh in our memory.
In 1948 the United Nations defined genocide as a crime against humanity subject to prosecution. The word "genocide" itself, is of recent vintage as history goes. It was coined by the lawyer Raphael Lemkin in 1943/4. He had the Armenian massacre, the intentional killing of up to 1.5 Armenians by the Turkish government during the First World War, very much in mind.