In its heyday, the Roman Empire stretched from the Firth of Forth to the cataracts of the Nile, from the Strait of Gibraltar to the Tigris. The secret of Roman rule, as Tacitus recognised, was integration: while other expanding empires merely conquered, Rome made its subjects Romans, at least according to the letter of the law. Nevertheless, the Roman world was a colourful cosmos: under the umbrella of the Pax Romana, people with the most diverse linguistic, cultural and religious backgrounds mostly lived together peacefully.