The US Congress' foreign affairs panel created a stir this autumn when it agreed that the deportation of 2 million Armenians from Turkey, between 1915 and 1923, was genocide.
Some asked why Congress was provoking America's ally over something that happened so long ago. The answer relates to the nature of genocide - an attempt to destroy a particular national, racial, religious or ethnic group in whole or in part - which is so awful that it is hard to comprehend and confront.
Germany's slow, painful recognition of the scale and evil of the Nazi genocide of Jews and gypsies has been exceptional. Other genocides, smaller in absolute numbers of victims but still horrifying, have not had the recognition they demand. Denial and self-deception are common. Only televised video footage of the 1994 Srebrenica mass killing of Muslims finally convinced many Serbs that it was true, and not propaganda.
In 1803, France attempted to exterminate the rebellious former slave population of Haiti, tying people of all ages and both sexes to cannon-balls and throwing them into the sea.
In 1904, general Lothar von Trotha, commander of German forces in South West Africa (today's Namibia), issued his 'extermination order' to kill all the Herrero people - 65,000 of whom were killed before the outcry forced him to stop.
The Turkish genocide of the Armenians was in two phases, both on a bigger scale than Srebrenica, Haiti or the Herreros. The lack of international response to the first encouraged the second.
In 1894, Armenians in the Turkish empire seeking equal civil rights with Turks held a demonstration in Istanbul. Turkish Sultan Abdul Hamid made this an excuse to use police and organised mobs to kill not just the demonstrators but at least 100,000 Armenians across Turkey - most dramatically at Urfa, where 3,000 were burned to death inside the cathedral.