From 27 BCE, when Octavian became Augustus, until roughly 180 CE, when Marcus Aurelius died — a span of two centuries — the Roman Empire administered the entire Mediterranean basin, half of Europe, and large parts of the Middle East as a single, internally peaceful, politically stable polity. A merchant could travel from Hadrian's Wall to the Euphrates speaking only Latin and Greek, paying in a single coinage, under a single legal system, on roads that the army maintained. No subsequent regime has matched it.
The Pax Romana was bought with several specific compromises. Augustus replaced the Republic's contested institutions with monarchical rule disguised as restored tradition. The army was professionalized and stationed at the frontiers, paid by the centre. Conquered elites were progressively made Roman citizens, with full legal rights, until — in 212 CE — citizenship was extended to every free man in the empire. Trade flourished because piracy had been suppressed and tariffs harmonized. Latin literature reached its peak; Roman law was codified; cities were founded with the same template — forum, baths, theatre, basilica — from York to Petra. The cost was the end of self-government for the populations involved, and a quiet centuries-long bargain in which security was traded for political voice. When the bargain frayed in the third century, the empire fragmented into civil war.
Every major imperial project since — Charlemagne, Byzantium, the Ottomans, the British, the American 'liberal order' — has measured itself against Rome and tried to extract its lessons. The durability of an empire turns out to depend less on military power than on legitimacy among the governed, which is a lesson regimes have repeatedly relearned.