In 208 AD, the Roman warrior emperor Septimus Severus arrived in Britain with 40,000 soldiers, intent on subduing the tribes inhabiting the northern part of the island. These tribes were part of the Caledonian confederacy, which occupied modern Scotland. But to the Romans, most everyone who lived outside the empire was a barbarian, full stop. So when Severus became frustrated by the Caledonians’ (sensible) refusal to submit to pitched battle, the emperor settled on another strategy, which we would now call genocide. In 210, he assigned the job of extermination to his son Caracalla, a mass-murdering lunatic who would later assassinate his own brother Geta in front of their mother. It likely was only Severus’ death in 211 that cut the operation short and saved Scotland from a complete holocaust.

Caracalla always is listed by historians among the worst emperors of Roman history. But tellingly, his attempted annihilation of the Caledonians isn’t typically cited in the historical bill of particulars. In ancient Rome, genocide was seen as an acceptable military tactic if it was directed at indigenous peoples. In a speech reportedly delivered to his troops before the Battle of Mons Graupius, the Caledonian chieftain Calgacus denounced the Romans thusly: “Robbers of the world, having by their universal plunder exhausted the land, they rifle the deep. If the enemy be rich, they are rapacious; if he be poor, they lust for dominion; neither the east nor the west has been able to satisfy them. Alone among men they covet with equal eagerness poverty and riches. To robbery, slaughter, plunder, they give the lying name of empire; they make a solitude and call it peace.” It is quite likely that the whole oration was invented by Tacitus. But invented or not, Calgacus’ words—especially the widely quoted ending—perfectly captured the mindset of genocidaires the world over, then and now, who always imagine themselves to be improving a society by annihilating it.

Today, we regard this mindset as sociopathic. But even into modern times, genocide often was justified as the cost of “progress.” In the 16^th century, Spanish colonists reduced the indigenous population of their Hispaniola colony from 400,000 to 200 within the space of a generation. The Belgian rape of Congo reduced a population of 20 million to about half that number. Stalin’s forced starvation of Ukraine killed about five million. In all cases, the killers believed that these genocides presented a net benefit to the civilized world. Hitler, who slaughtered six million Jews, thought that the entire planet one day would lionize him for ridding the world of what his diseased and evil mind conceived as a uniquely destructive pestilence upon humanity.