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The Pax Romana runs from 27 BC, when Augustus became Rome’s first emperor, to the death of Marcus Aurelius in 180 AD. Under Trajan around 117 AD the empire held some 70 million people, about a third of humanity. Yet historians such as Walter Goffart note that the Cambridge Ancient History volume titled The Imperial Peace is full of revolts, Persian wars and frontier fighting. Arthur M. Eckstein argues the peace was relative and arrived in stages, while Arnaldo Momigliano defined Roman peace as enemies beaten until they could no longer resist.

After Actium in 31 BC, Augustus built a disguised military regime while calling himself only princeps, first citizen, and sold peace through the Ara Pacis, the Secular Games of 17 BC, coins stamped Pax and the closing of the gates of Janus. The phrase Pax Romana first appears in Seneca the Younger in 55 AD. Safe seas boosted trade alongside Han China’s Pax Sinica, and the ius gentium gave a shared legal code. Raymond Aron’s theory of imperial peace explains the First Jewish-Roman War, and Cassius Dio called the fall after Commodus a descent from gold to iron and rust.

• In his Res Gestae Augustus boasted of closing the Temple of Janus three times.
• Scholars date the third closure to 13 BC, after campaigns in Hispania and the Alps with Agrippa.
• Rome’s ships brought back silk, gems, onyx and spices from the east.
• Circuit judges reached provincial towns on a fixed schedule, curbing vigilante justice.
• Aron held that imperial peace became civil peace only as memories of independence faded.