When the Romans were unable to reach an accomodation with an enemy, and had found the enemy so intractable as to be incapable of keeping a peace agreement, they would, after conquering, kill every living thing in the enemy's territory: men, women, children, dogs, horses - everything.
The Romans did not do this out of savagery, but because they had determined over the course of hundreds of years that such measures were the only effective means of causing barbarians to fear Rome enough that they would stop committing depradations on the fringes of the Empire.
That was true then, and it's true now.