Reddit reviews Caesar's Commentaries. The Complete Gallic Wars. Revised.: Revised Edition

During the height of its power, starting with Julius Caesar's campaign in Gaul, the Roman Republic and early Empire had a stated policy of "Murum Aries Attigit", which translates to "Until the Ram Touches the Wall". In short, at the very start of the siege, the Romans would issue (usually fairly favorable) conditions for surrender and state that the offer was only good until their battering rams touched the walls of the city. Once that happened, no surrender would be permitted or accepted, and it usually meant the city was pillaged, slaughtered, burned to the ground, and occasionally plowed into the earth so it was nothing but a scorched patch of earth.

This show of thorough violence made it very likely that the next village they laid siege to would be more inclined to surrender early, and save the Romans the time and trouble of destroying them.

Source: Caesar's Commentaries on the Gallic Wars