I spoke too loosely in describing the extirpation of the Albigenses as the first genocide in European history. Â It would have been more precise to say “post-Classical European history,” as a Jeff Weintraub points out:
One passage in Caesar’s Commentaries on the Gallic Wars has always struck me as particularly chilling. In Book 6 Caesar describes how he defeated a string of revolts, culminating in the great revolt led by Vercingetorix. One of those was the revolt of the Eburones, a Belgic tribe, led by a certain Ambiorix. They managed to ambush and either wipe out or devastate several units of Roman soldiers. When Caesar arrived with his main army, looking for revenge, Ambiorix fled across the Rhine, and the rest of the Eburones scattered into the forests for safety.  Caesar writes:
There was, as we have above observed, no regular army, nor a town, nor a garrison which could defend itself by arms; but the people were scattered in all directions. Where either a hidden valley, or a woody spot, or a difficult morass furnished any hope of protection or of security to any one, there he had fixed himself. These places were known to those who dwelt in the neighborhood, and the matter demanded great attention, not so much in protecting the main body of the army (for no peril could occur to them altogether from those alarmed and scattered troops), as in preserving individual soldiers; which in some measure tended to the safety of the army. Â […..]
Caesar dispatches messengers to the neighboring peoples [civitates]; by the hope of booty he invites all to him, for the purpose of plundering the Eburones, in order that the life of the Gauls might be hazarded in the woods rather than the legionary soldiers; at the same time, in order that a large force being drawn around them, the race and name of that people may be annihilated [stirps ac nomen civitatis tollatur] for such a crime. A large number from all quarters speedily assembles. […..]
We still have the name of the Eburones, thanks to Caesar. But as for the Eburones themselves, my impression is that as a people they were indeed effectively destroyed, and their territory was taken over by other neighboring tribes. (Or perhaps Caesar was boasting about a genocidal mass murder he didn’t fully accomplish? Either way, as I said, I’ve always found that passage a bit chilling.)
We should note that this particular episode had nothing to do with religion. Caesar was mostly trying to make a point.
Also note that De Bello Gallico was not just an historical account.  It was also a public-relations document highlighting Caesar’s military triumphs on behalf of the republic. It might even be called a campaign document, since top figures in the Roman political elite were, effectively, involved in a never-ending “campaign.” In that context, the matter-of-fact tone in which Caesar announces his intention that “the race and name of that people may be annihilated” is especially striking. There is no trace of embarrassment, euphemism, or circumlocution about it.
So it’s fair to say that there has been some moral progress in 2000 years. Â At least, modern advocates of genocide tend to use euphemisms.