At its height, under Trajan in AD 117, the Roman Empire ran from the north of Britain to the Persian Gulf and from the Rhine and the Danube to the edge of the Sahara. Every shore of the Mediterranean was Roman, which is why the Romans could call it Mare Nostrum, our sea, without exaggerating. Somewhere between fifty and sixty million people lived inside it, perhaps a fifth of everyone alive at the time.
How it got so big
It was not planned. Rome spent the Republic acquiring territory one war at a time, usually after deciding that a neighbour was a threat, and then finding that the next neighbour along was a threat too. Sicily came first, in 241 BC, at the end of the First Punic War: the island became Rome's first province, and its grain fed the city for centuries after. Spain, Greece and North Africa followed in the second century BC, Gaul under Caesar, Egypt under Augustus, Britain under Claudius, and Dacia, the last great conquest, under Trajan himself.
How it was run
The empire was a patchwork of provinces, each with a Roman governor, a capital, a garrison if it needed one and a tax bill. Augustus split them in two. The settled, peaceful provinces, Sicily among them, stayed with the Senate and were governed by former consuls and praetors chosen in the old way. The frontier provinces, where the legions were, he kept for himself and ran through his own appointed legates, which is the quiet arithmetic of how an emperor stays an emperor: whoever names the generals keeps the army. The map at the top of this page colours the two kinds differently.
Beyond that the empire governed remarkably lightly. Cities ran their own affairs through local councils, kept their own gods and mostly their own languages, and the central state asked for taxes, order and loyalty, in roughly that order. Latin was the language of the west and of the army; Greek remained the language of the east, and educated Romans spoke both.
The army on the edges
Holding all of it took around thirty legions of roughly five thousand citizen soldiers each, with as many again in auxiliary units raised from the provinces. Almost none of them were in Italy. They sat on the frontiers in permanent fortresses along the Rhine, the Danube, the Euphrates and the desert edge, and behind lines like Hadrian's Wall in Britain, built from AD 122. A legionary signed on for twenty five years. If he lived, he retired with land or a cash bounty, and, if he had joined as an auxiliary, with Roman citizenship for himself and his children, recorded on a military diploma.
Roads, law and money
What held the empire together was less the army than the things the army protected. More than fifty thousand miles of paved road ran out from Rome, built first for soldiers and used by everyone. One coinage, the silver denarius and the gold aureus, was good from Britain to Syria. Roman law followed the roads, and after AD 212 nearly every free person in the empire was a Roman citizen under it. A merchant could sail from Spain to Alexandria without crossing a border, paying a pirate or changing his money.
Two halves, and an end that took a thousand years
An empire that size was too big for one man to watch, and in the third and fourth centuries it was increasingly run as two halves, a Latin west and a Greek east, with the capital of the east at Constantinople from 330. The western half came apart in the fifth century under pressure from the peoples on its borders; the last western emperor was deposed in 476. The eastern half, which called itself Roman to the end, lasted until 1453.
The empire you can still see is mostly the part that outlived it: the roads, the Latin under every romance language, the law books, the calendar, the cities sited where Roman camps and colonies were, and the eagle that a legion carried at its head. For the city at the centre of all of it, see where Rome sits, and for how it got there, a short history of Rome.