Of the events in history that are typically labeled as genocide, the first in modern times began in 1915. This was the genocide of Armenians by the Ottoman Empire. This is considered to have been an instance of genocide by most historians, but we must be aware that Turkish historians and some others hotly contest this characterization.
The genocide of the Armenians (along with similar actions against some other ethnic minorities) began as WWI was going badly for the Ottomans and as Turkish nationalism was rising within the empire. Anger over setbacks in the war, as well as the growing sense of nationalism among the “Young Turks,” led to the beginning of a program of deporting Armenians from the area that is now Turkey and to the killings of many of these Armenians. Most scholars place the death toll at somewhere between 1 and 1.5 million Armenians. This is generally considered to have been the earliest modern act of genocide, one which began in 1915.