In the 13th century, forces loyal to Pope Innocent III laid siege to the stronghold of Montsegur, home of the Cathar sect. Setting out to annihilate the sect, one commander is reported to have said: “Kill them all and let God sort them out.”

This is one example that JM Berger discusses in his book Extremism, where he defines an extremist ideology as something perpetually opposed against an ‘other’ until that other is destroyed. The crusade against the Cathars was an extremist one, so were the Romans’ feelings towards Carthage, and those of the Jewish Sicarii towards their Roman occupiers.