Edward Gibbon on the Roman destruction of Seleucia, its own ally

The sack and conflagration of Seleucia, with the massacre of three hundred thousand of the inhabitants, tarnished the glory of the Roman triumph [over enemy city, Ctesiphon,  just across the Tigris river].

    --Edward Gibbon (1737-1794) in The Decline and Fall of the Roman Empire (published 1776-1788), Vol. 1, ch. VIII. At the time, Seleucia was one of the world's largest cities, and an ally of Rome.