The Roman Empire reached its largest territorial expanse during the reign of Trajan (AD 98–117), encompassing an area of about 5 million sq km (1.93 million sq m).
The maps below show The Roman Empire at its territorial height.
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The topography of the Roman Empire
The Roman army mainly was infantry based, which was great in hilly or mountainous terrain but at a critical disadvantage to cavalry on the open flats. As a result, the Roman Empire’s frontiers mainly situated on one of the following:
- Coast (the entire Mediterranean basin, Iberia, most of northern and western Europe)
- Mountains (Balkans, Alps, north-western Africa, Anatolia in modern Turkey, Middle East)
- Desert (northeastern Africa and the Arabian peninsula)
Any of the above is an efficient barrier to the tactical movement and logistical mobility and supply of large cavalry-based armies, the curse of the Roman army during its existence, and the reason for some of its most dramatic losses, from Cannae to Carrhae.
Behind tactical considerations, swaths of rough territory also give a natural geographic constraint to population density overall. On such edges, the Romans were less likely to have a lot of the enemy population dwelling close to the border, to begin with, suggesting a thinly-stretched garrison could protect a large part of the border as far as local border clashes went, while also having more time to prepare a response to any large attacks before they could breach the guarded perimeter.
Below is the fantastic relief map of the Roman Empire created by @mappdoutofficial.
Whenever the Romans had an extent of the border that didn’t meet at least one of the above criteria, they had a permanent source of trouble that they couldn’t permanently resolve. So any conquest just pushed the boundary a bit further without really excluding the threat, only over-stretching their supply lines in the process. In the Eastern territories of the Empire, almost all of those affected losses to cavalry and especially cavalry archers, so the final abandonment of most of Mesopotamia east of Anatolia and the Levant. The other territories with flat, poorly defensible land boundaries linked with powerful enemies were the extents of Dacia and Germania, which unsurprisingly are the positions from where Rome was finally invaded and defeated by the Huns and Goths. But unlike Mesopotamia, Roman Empire couldn’t retire from these boundaries without compromising the defending line and territorial integrity of the whole Empire, so they had no option but to stay.
The just notable exemption to this rule is Britannia, where, opposed to the general style of the Romans, they did well in the English plains yet couldn’t capture the Scottish Highlands. But in the whole system of things, this exemption is rare enough to explain the rule and was possibly driven by logistical and economic factors (too far-off and not deserving the trouble) rather than military considerations.
The map below shows the Roman Empire in 117 AD at its territorial height at Trajan’s death.
The Roman Empire in 117 AD
The map below shows the Roman Empire, at its height, overlying on current political borders.
The Roman Empire, overlying on modern borders
The Roman Empire at its peak encompasses about 5 million sq km (1.93 million sq m). Below the interesting map giving created by Arnold Platon, the answer to the question “When standing in Rome, how much Empire is there in each direction?”
How big was the Roman Empire without the Mediterranean Sea?
A period of progressing trouble and deterioration of the Roman Empire began with the rule of Commodus (177–192).
After the military and political crises, the Roman empire was ruled by many emperors who shared control over the Western Roman Empire and the Eastern Roman Empire.