Borders of the Roman Empire
The borders of the Roman Empire stretched from the deserts of Arabia to northern Britain. Hadrian's Wall alone required a garrison of fifty thousand soldiers. What kept such a vast perimeter together depended entirely on where you were standing. The decisions that drove those choices, and the pressures that forced Rome to revise them, run through every century of Roman history. The very Latin word the Romans used for frontier turned out to mean several different things at once.
The Latin noun limes carried at least five distinct meanings in classical usage, none of which corresponded exactly to "frontier". It could mean a path marking off the boundaries of fields, or a boundary line, or any road or path. It could also refer to a channel such as a stream, or simply a distinction between two things. After the third century, Roman administrators gave it a more specific meaning: a military district commanded by an officer called a dux limitis. Modern scholars later adopted limes as a general term for the Roman frontier. The Romans themselves never used the word that way.
In Britannia, the empire built two walls one behind the other. In Mauretania, a single wall ran with forts placed on both sides of it. In Syria and Arabia Petraea, there was no continuous wall at all; a net of border settlements and forts covered the terrain instead. In Dacia, between the Black Sea and the Danube, the Limes Moesiae was organized around two or sometimes three parallel lines of earthen rampart. The system included a Great Camp and many smaller outposts spread throughout.
The Antonine Wall, constructed between 142 and 144, extended Rome's defensive presence in Britain. What became of it in the following decades shaped how historians have understood the empire's northern limits.
The Rhine and Danube rivers defined most of the empire's continental border, running through central Europe as a natural line the legions could defend. The province of Dacia, which corresponds to modern Romania, lay entirely on the far side of the Danube. It was one of several places where the empire extended beyond what the rivers could define. The province of Germania Magna occupied the land between the Rhine, the Danube, and the Elbe. It survived only three years after its creation, lost as a result of the Battle of Teutoburg Forest.
In Britain, the Antonine Wall was abandoned by 164. It was briefly reoccupied in 208, under the reign of Septimius Severus. What the eastern border required was something none of the solutions tried in the north could provide.
The Sasanian Empire succeeded the Parthians in 224-226 and was recognized as one of the leading world powers of its age. That placed it alongside Rome, and later the Byzantine Empire, as a rival of comparable standing for more than four hundred years. The Parthians, who preceded the Sasanians, had ruled most of Greater Iran, covering what is now Iran, western Iraq, Armenia, and the Caucasus. The eastern border changed shape many times over those centuries, driven by pressure from empires that matched Rome in power and reach.
At the empire's southern edge, the limiting factor was not a rival army but geography.
Septimius Severus reached the Garamantian capital of Garama in AD 203, briefly holding a military presence there. Most of the campaign's initial success was achieved by Quintus Anicius Faustus, the legate of Legio III Augusta. Severus dramatically expanded the Limes Tripolitanus during this period. Following his African campaigns, the Roman Empire may have reached its greatest extent under his reign, encompassing two million square miles.
South of the Mediterranean coast, the Romans controlled mountain ranges farther inland. Beyond those, the Sahara in North Africa and the deserts of Arabia formed a natural barrier against further expansion. From the Atlantic Ocean to Egypt, Rome organized the African frontier into many separately named zones. These included the Limes Arabicus, also known as the Limes Uranus, and the Limes Mauretaniae. They also encompassed the Fossatum Africae, the Fossa Regia, the Limes Tripolitanus, and the Limes Numidiae. In the south of Mauritania Tingitana, a limes was built in the third century just north of the area of modern Casablanca. It ran near the town of Sala and extended to Volubilis.
The Romans twice attempted to occupy the Siwa Oasis and eventually used it as a place of banishment instead. Along the Nile, Roman control extended far into Africa, reaching the towns of Syena, Berenice, Hyerasykaminos, and Qasr Ibrim. Qasr Ibrim, the southernmost point of all, lay close to the modern border between Egypt and Sudan. The exact period in which each of those towns represented Rome's final frontier has never been established.
Common questions
What were the borders of the Roman Empire made of?
The borders of the Roman Empire combined military roads and linked forts, natural frontiers such as the Rhine and Danube rivers, and man-made fortifications. In some provinces a continuous wall was built; in others such as Syria and Arabia Petraea, a net of border settlements and forts served instead.
How large were the borders of the Roman Empire at their greatest extent?
The Roman Empire may have reached its greatest territorial extent under Septimius Severus, encompassing two million square miles of territory.
What was Hadrian's Wall and what role did it play in the borders of the Roman Empire?
Hadrian's Wall was a defensive fortification constructed in 122 to protect the province of Britannia from the Caledonians, holding a garrison of fifty thousand soldiers. It was one of two walls the Romans built in Britain; the second, the Antonine Wall, was constructed between 142 and 144.
What did the Latin word limes mean in the context of Roman borders?
In classical Latin, limes had several meanings, including a path, a boundary line, any road or path, a channel, or a distinction between two things. After the third century it became an administrative term for a military district commanded by a dux limitis. Modern scholars use it as a general term for the Roman frontier, a usage the Romans themselves never adopted.
What empires did the Roman Empire face along its eastern borders?
Along its eastern borders, the Roman Empire faced the Parthian Empire and then the Sasanian Empire, which succeeded the Parthians in 224-226. The Sasanians were recognized as a leading world power alongside Rome and the Byzantine Empire for more than four hundred years.
How far south did the borders of the Roman Empire extend in Africa?
The Roman Empire controlled the North African coast from the Atlantic Ocean to Egypt, with the Sahara and the deserts of Arabia forming the southern natural barrier. Along the Nile, Roman control extended to Qasr Ibrim, the southernmost point of the empire, near the modern border between Egypt and Sudan.