It started around World War I and continued for a bit after. It was when the fading Ottoman Empire took advantage of a crack down on Armenian separatists to pretty much undertake a systematic genocide of the Armenian people, largely because they were a sizable Christian minority in a predominantly Muslim country. They employed the usual deportations, deprivations, and concentration camps. There's an argument that the Nazis got ideas from them, given the long-standing link between German intellectuals of the pan-Aryan movement and Ottoman-Turkish thinkers.
Turkey, the country that eventually emerged at the center of the collapsed Ottoman Empire, has consistently denied that there was any attempted genocide, while pretty much the rest of the world agrees there was, and that it is more or less the first attempted one in modern history. Turkey still tends to overact to anyone who talks about it, arresting them for "profaning the Turkish state," i.e., making it look stupid, which is still a crime in Turkey.
That's what I remember from college. Correct me if I'm wrong.