Rome's famous two centuries of peace were quieter than before, not peaceful
The Pax Romana is usually dated from Augustus in 27 BC to the death of Marcus Aurelius in AD 180, one of history's longest stretches of relative calm. Yet it included revolts and wars with Persia, and one historian observed that a standard history volume on the era, titled The Imperial Peace, contains very little peace.
Romans did not think of peace as simply the absence of war. For them it was the rare condition in which every enemy had been crushed and could no longer resist. Augustus had to persuade a warrior society that prosperity without fighting beat the glory and loot of risky campaigns, and he did it largely through propaganda: ceremonially shutting the doors of the Temple of Janus, a sign Rome was at peace, in 29 BC and 25 BC, and probably again in 13 BC when the Altar of Peace was commissioned. Later emperors stamped Pax on coins and sponsored poets who praised it.
His political fix mattered as much as the messaging. Lacking any model for successful one-man rule, he bound Rome's leading generals into a coalition and stood at its head as first citizen, removing the risk of renewed civil war after his victory at Actium in 31 BC.
Dates are disputed. Some scholars trace the calm back to the second century BC, when Rome had eliminated its serious rivals, and note that it reached different regions at different times: Sicily after 210 BC, Italy after 200 BC, much of Iberia after 133 BC. The phrase itself first appears in the writing of Seneca the Younger in AD 55.
Whatever the dates, the era was prosperous. The empire reached its largest extent under Trajan in 117, and its population peaked at about 70 million, a third of humanity. Mediterranean trade flourished, with ships sailing east for silk, gems and spices, helped by the parallel stability of Han China. After Marcus Aurelius, the historian Cassius Dio saw a slide from gold to iron and rust.
Source: Pax Romana
