The Ra'zac War was a bloody conflict between the Dragon Riders and the Ra'zac and could possibly be described as genocide. The Ra'zac arrived in Alagaësia from an unknown land, thought to be that of Humans (they seemingly followed them to Alagaësia). The Ra'zac were naturally suited to hunting Humans and loved the taste of their flesh. The Ra'zac wreaked havoc among Humans. Finally, the Dragon Riders stepped in to try to rid the world of these ruthless monsters. The Riders eventually succeeded in killing all but two of them before the Fall. An actual name for this conflict has never been given in the Cycle.
This conflict, while seldom mentioned, has created an amount of controversy. As stated above, the war could be interpreted as an act of genocide. Fans have felt that the Dragon Riders, a supposed order of good, should have known better than to exterminate an entire race. Arya has claimed that the Ra'zac are inherently evil, although there is no proof to back it up.
It could be that the Riders only exterminated the Ra'zac who had immigrated to Alagaësia, not the ones who were still in their homeland.