The werewolf genocide was an attempt by the Volturi to wipe out the Children of the Moon, started by Caius, after he was nearly killed by one centuries ago. Caius became terrified of them, and convinced the rest of the Volturi to help him drive them into extinction.
The actual events are largely unrecorded, but the Volturi nearly succeeded in making the werewolves extinct in Europe and Asia. It is unknown if they are found anywhere else, but it is implied that at least a small number of them still exist since James, Victoria and Laurent hunted one down in Siberia.
In late December 2006, Caius made the mistake of assuming the Black and Uley packs as the Children of the Moon when they changed into wolves; Aro corrected him by saying they were merely shape-shifters.