How big can the Roman Empire get?

[Mr_Fanboy]

Here is a topic that I have not seen discussed in a while: with any point of divergence after the beginning of the Imperial era in 27 BC, how much territory can realistically fall under Rome’s rule? Germania? The entirety of the British Isles? The entirety of Europe? The Arabian Peninsula? Ethiopia? Persia? Even further east and south?

Secondary question: what would be the economic, religious, political, and social impact of the Roman Empire achieving whatever you believe to be its maximum border, both on the conquered territories and the core Roman provinces? How long would it last until partial or total collapse, or at least serious territorial shrinkage?

 

[Atamolos]

It all depends on timeframe. The traditional territorial maximum of the Roman Empire is depicted as the empire at the time of Trajan's death, however, most of his Eastern conquests were only occupied for 3-5 years, which is hardly a "glorious apex of the empire". No empire exists forever, and the Roman Empire is no different. It cannot reasonably hope to hold onto the entirety of it's territory for anywhere near the same length of time, and throughout the long decline of the Western Empire, it shed it's territories at various speeds. Does Dacia count as being "under Rome's rule", even though it was only occupied for a paltry 150 years? What about Caledonia? It was subdued and conquered by Agricola, as described by Tacitus, but Roman administrators never held much sway on the northernmost reaches of the island, so does Caledonia count or not?

My point is that literally any territory could "realistically" fall under Rome's "control", but without defining what you mean by "realistic" and "control", there's no way to provide a meaningful answer. If what you mean is, "How many territories may plausibly be considered as part of the core Roman Empire as defined by the parts centrally administered during the imperial 'golden age' (traditionally the Antonine period)?" then we have a question with well-defined parameters to which we can hash out an answer, but without being more specific there's not much else to say.

So, to address my re-framed question, a whole bunch of territories could plausibly be brought under Roman control for a century (probably all of Brittania, part of southern Germany, the rest of Dacia, and maybe part of Morocco, Nubia, and Arabia), but each of these would present serious administrative challenges without clear strategic benefits, so the incentive to conquer these areas simply isn't there. Even the conquest of what we see as the "early Roman Empire" was only made possible due to the centralization and strategic planning of Augustus, and I have my doubts that any comparable centralized administration would find rationale to conquer such remote areas.

 

[Mr_Fanboy]

My point is that literally any territory could "realistically" fall under Rome's "control", but without defining what you mean by "realistic" and "control", there's no way to provide a meaningful answer. If what you mean is, "How many territories may plausibly be considered as part of the core Roman Empire as defined by the parts centrally administered during the imperial 'golden age' (traditionally the Antonine period)?" then we have a question with well-defined parameters to which we can hash out an answer, but without being more specific there's not much else to say.

Fair enough. To be more specific, how much territory can be conquered and then Romanized to at least the extent of, say, Roman Britain in our timeline before the empire started to fall apart? I mean this in terms of culture, standard of living, and general integration with the wider empire.

 

[Socrates]

The Ottomans had top rate logistical capabilities and never managed to push further East than the furthest stretches of Anatolia, so I think that is your Eastern boundary. The Sahara is pretty impossible to project power across in the pre-industrial age, so that is your Southern Maximum. For the North, it depends how long the Empire exists for. The main challenge is that most of the ground is heavily forested, making it hard to build urban societies which prevents power projection. However, the more centuries pass, the more wood will get cut down. Perhaps you could get to the Vistula. With enough centuries and the right advances in ship technology, I think you could include the British Isles, southern Scandinavia and the Baltics.

 

[Rèxīn]

After my first genuine interest in history (right after painting Paradox maps with the purple), I had and asked the same questions and when I did some research, oh man was I surprised. First off, I'd agree that a Roman Western Europe could have been possible since the Romans have operated across the Rhine and half of Britain was already conquered, obvious POD would be the Julio-Claudian dynasty performing better, such as a long lived Drusus / Germanicus which butterflies the depressed Tiberius and Sejanus.

The rest seems implausible because logistics, which as I say it brings shame to my Romanophilia. At the height of the Roman Empire under Trajan, its size as per Wikipedia is 4.4 million sq. km. For comparison:

Ming Dynasty - 6.5 million sq. km
Eastern Europe minus the former Roman territories - 5.4 million sq. km
Arabian Peninsula - 3.3 million sq. km
Sudan - 1.88 million sq. km
Horn of Africa - 1.88 million sq. km
Iran - 1.65 million sq. km
Iraq - 0.44 million sq.km

Not only do the Roman armies have to march hundreds and thousands of miles to conquer people who will prefer to die as barbarians rather than become part of the glorious Roman Empire, but they also have to traverse the different extremes of climates which is not good for morale or loyalty. After that, the emperor has to somehow effectively maintain its authority to prevent revolts and rebellions of disgruntled generals, which is impossible.

Second question is harder to answer without specifics but in my scenario, had the Julio-Claudian dynasty managed to survive longer and tie the emperor title to their blood and the empire falls, the emperor in title-only could have been a kingmaker and/or divine leader instead of the pope, similar to Japan. Or if the empire survives, I see it carrying its Roman antiquity values with a divide slightly favoring a Royalist faction more than a Republican one due to Divine (Saint?) Caesar / Divine Augustus dogmatism much to the disdain of monotheist purists like Christians and Jews.

 

[Karolus Rex]

Here is a topic that I have not seen discussed in a while: with any point of divergence after the beginning of the Imperial era in 27 BC, how much territory can realistically fall under Rome’s rule? Germania? The entirety of the British Isles? The entirety of Europe? The Arabian Peninsula? Ethiopia? Persia? Even further east and south?

Secondary question: what would be the economic, religious, political, and social impact of the Roman Empire achieving whatever you believe to be its maximum border, both on the conquered territories and the core Roman provinces? How long would it last until partial or total collapse, or at least serious territorial shrinkage?

Is this territory they are supposed to hold for a long period or simply lands they can conquer but whose control would be short lived?

I can't talk much about the political and social impact, I suspect an even greater empire would cause even more internal strife as the legions would be even more isolated from their nominal leaders in Rome.

On the economics, per Ward-Perkins research on the late Empire, there was a downside of the economical complexity of the Roman economy in the Western provinces, Italy having already been on a small downside trend and with the other western provinces starting to suffer the same effects in the mid 300's, adding Germania and the rest of the British Isles will not help in the eventual economical decline the Empire would suffer, neither were particularly rich or prosperous regions and would, in my view, would simply be weights on the Roman economy. The Eastern economy entered its downturn phase later than the west, and I feel that the distance between the POD date and the Eastern downturn phase is too big to be able to do any predictions on how it would evolve.

 

[Magnum]

Is this territory they are supposed to hold for a long period or simply lands they can conquer but whose control would be short lived?

I can't talk much about the political and social impact, I suspect an even greater empire would cause even more internal strife as the legions would be even more isolated from their nominal leaders in Rome.

On the economics, per Ward-Perkins research on the late Empire, there was a downside of the economical complexity of the Roman economy in the Western provinces, Italy having already been on a small downside trend and with the other western provinces starting to suffer the same effects in the mid 300's, adding Germania and the rest of the British Isles will not help in the eventual economical decline the Empire would suffer, neither were particularly rich or prosperous regions and would, in my view, would simply be weights on the Roman economy. The Eastern economy entered its downturn phase later than the west, and I feel that the distance between the POD date and the Eastern downturn phase is too big to be able to do any predictions on how it would evolve.

Taking the entirety of the British isles would have allowed Rome to substantially reduce the garrison stationed there to a single legion or thereabouts.

To the OP, with a PoD during the First Punic War where Rome takes Carthage, you could get quite far. With 27 BC, it's a lot more iffy to expand it all that much in just two centuries

 

[Karolus Rex]

Taking the entirety of the British isles would have allowed Rome to substantially reduce the garrison stationed there to a single legion or thereabouts.

To the OP, with a PoD during the First Punic War where Rome takes Carthage, you could get quite far. With 27 BC, it's a lot more iffy to expand it all that much in just two centuries

Taking, holding all of the British Isles and keeping them pacified would, IMO so take it as you wish, have required a significant addition to the garrison, not to a downside of its size, as they don't just have to conquer it, they have to hold it, they need to keep the tribes under control and peaceful, that would mean forts and garrisons on the main settlements, roads and passages, while needing a strategic reserve near the governor in case there's a revolt from the tribes, in addition to the forces that had to be deployed in the Britannia to do the same job there.

 

[Tonifranz]

Taking, holding all of the British Isles and keeping them pacified would, IMO so take it as you wish, have required a significant addition to the garrison, not to a downside of its size, as they don't just have to conquer it, they have to hold it, they need to keep the tribes under control and peaceful, that would mean forts and garrisons on the main settlements, roads and passages, while needing a strategic reserve near the governor in case there's a revolt from the tribes, in addition to the forces that had to be deployed in the Britannia to do the same job there.

Maybe not. Many provinces the Romans conquered later on did not need many legions to cover, even if earlier, they did. Just look at Spain. Earlier, when it was bordering unconquered regions, it has sizeable number of legions stationed. But after all the last tribes were conquered by Augustus, the Romans got away with just one legion, or even less, for most of the time period of the Empire.

And look at NOrth Africa outside Egypt. Only one legion was deployed during the High Empire.

Generally, if they conquered all of the British Isles, it would be the same. They got away with holding Spain with almost no revolts for four centuries with a single legion or two. No reason they can't get away with other pacified regions.

 

[Karolus Rex]

Maybe not. Many provinces the Romans conquered later on did not need many legions to cover, even if earlier, they did. Just look at Spain. Earlier, when it was bordering unconquered regions, it has sizeable number of legions stationed. But after all the last tribes were conquered by Augustus, the Romans got away with just one legion, or even less, for most of the time period of the Empire.

And look at NOrth Africa outside Egypt. Only one legion was deployed during the High Empire.

Generally, if they conquered all of the British Isles, it would be the same. They got away with holding Spain with almost no revolts for four centuries with a single legion or two. No reason they can't get away with other pacified regions.

Spain had a large presence of auxiliary units spread across the province garrisoning forts, current research on just the soldiers that were active on the region of north western Iberia, galicia and western asturias, during the first century CE, based on new archeological findings, place the number of active troops on the region on as high as 20,000, again spread across forts as they had to safeguard the control of strategic locations.

And for each legion, you had large forces of auxiliary cohorts deployed in the regions to man fortifications and strategic locations.

To control and pacify a region like Scotland would had required a large influx of auxiliary forces and vexillationes to safeguard Roman control over the region.

 

[Tonifranz]

Spain had a large presence of auxiliary units spread across the province garrisoning forts, current research on just the soldiers that were active on the region of north western Iberia, galicia and western asturias, during the first century CE, based on new archeological findings, place the number of active troops on the region on as high as 20,000, again spread across forts as they had to safeguard the control of strategic locations.

And for each legion, you had large forces of auxiliary cohorts deployed in the regions to man fortifications and strategic locations.

To control and pacify a region like Scotland would had required a large influx of auxiliary forces and vexillationes to safeguard Roman control over the region.

At the beginning of course they would need many troops. After a century or so of continuous Roman rule, if they are not on a frontier post, they would reduce the number of troops substantially. If you look at legion deployments, most of the legions were on the Danube, Rhine, and Syria. How many legions are actually in Greece, Macedonia, in Eastern Anatolia, in Italy, in Narbonensis Gaul, in Carthage, in Sicily, areas that were under centuries of Roman rule? And in Spain too?

Look at North Africa. Almost very few troops in comparison to the East and North. And 20,000 troops are nothing to the 400,000 total that the Romans has. Most of the troops would still be stationed on the Rhine, Danube, and Syria.

And yeah, my research indicates that in Spain, in comparison to during the time of Augustus, where there were three, four, even seven legions stationed during the conquest of the Cantabrians, during the time of Hadrian, only one legion was stationed and an equivalent auxiliaries.

So pacified areas with no land frontier would actually require less troops.

 

[Derek Pullem]

Spain had a large presence of auxiliary units spread across the province garrisoning forts, current research on just the soldiers that were active on the region of north western Iberia, galicia and western asturias, during the first century CE, based on new archeological findings, place the number of active troops on the region on as high as 20,000, again spread across forts as they had to safeguard the control of strategic locations.

And for each legion, you had large forces of auxiliary cohorts deployed in the regions to man fortifications and strategic locations.

To control and pacify a region like Scotland would had required a large influx of auxiliary forces and vexillationes to safeguard Roman control over the region.

That is true but the logic was that auxillia from other provinces would be used and the auxillia raised from Scotland get deployed to Germany or some other remote place. So it's not like there is a strong military agitating against the Empire in the border provinces. Of course when the Empire invited Fedoerati en masse later on it was a disaster.

Having said all of that, the logistics of expanding the Empire too far away from the Mediterranean coastline or major rivers do put a hard limit on the effective control of the Roman Empire. If the Romans decided to colonise the Black Sea coastline that would be possible (but not much in the way of resources to justify it). Similarly they could decide to build a Suez canal (or dredge the existing one) to increase Roman influence in East Africa but again the question is what is there to gain there. Trade doesn't require Empire.

 

[Fabius Maximus]

If you butterfly away the Roman defeat at Teutoberg then Augustus' German province would probably survive. Eventually you might see Roman rule extend over all of modern Germany.

 

[John7755 يوحنا]

Perhaps the following:

Italia
Greece
Africa
Mauritania
Hispana (all of the peninsula)
All of Gaul
Ireland/Hibernia
Britain + Scotland/Pict country
Both Germanias, extend to the Elbe
Asia Minor
Anatolia
Pontus
Egypt
Syria
Cyprus
Palestine
Phoenicia
Assyria
Babylonia (new border at Zagros mountains)
Jordan
Armenia
Bosporus/Crimea
Dacia
Thracia
Illyria
Pannonia
Noricum-Rhaetia
Colchis-Iberia
Mannaea

This would be in my opinion, the possible maximum expansion of the empire at a single moment. They may not keep it, but regardless, this could potentially be performed. A reminder, few polities in history have ever ruled a vast empire beyond fantastic rulers and periods of golden age. Only progressively did the Chinese states slowly integrate much of East Asia into their realm and sphere. Achaemenid Shahs did so only by way of the prior period which saw the subjugation of most of the vast region aside from Egypt and Anatolia.

 

[Magnum]

Much of Rome's conquests in the east relied on superimposing themselves atop the existing hellenistic administration.

Problem is that more than 100 years had passed since the last major Seleucid effort to contest Mesopotamia. If Augustus decides to invade east instead of halving the army in 27 BC, he's going to have a hard time. Granted, he could also redeploy forces from Spain, but it would still be hard, and the Romans would have little in the way of support in Mesopotamia in order to hold it long term. If (and that's a big if) they CAN hold it, then nominally annexing the Arabian peninsula after taking key settlements wouldn't be all that difficult, though it would be a large headache for little gain.

 

[John7755 يوحنا]

Much of Rome's conquests in the east relied on superimposing themselves atop the existing hellenistic administration.

Problem is that more than 100 years had passed since the last major Seleucid effort to contest Mesopotamia. If Augustus decides to invade east instead of halving the army in 27 BC, he's going to have a hard time. Granted, he could also redeploy forces from Spain, but it would still be hard, and the Romans would have little in the way of support in Mesopotamia in order to hold it long term. If (and that's a big if) they CAN hold it, then nominally annexing the Arabian peninsula after taking key settlements wouldn't be all that difficult, though it would be a large headache for little gain.

Many of these Seleucid era policies and administration were still in effect under the Arsacid and even under the Sassanids. I am not sure that Rome effectively utilized the free city systems that the Seleucids made their lasting effect upon later Arsacid-Sassanid empires. But it is surely the case, the Arsacids built much of their own ruling dogma upon the Seleucid era motifs and systematics, rather than on the prior Achaemenid period.

My point, it is Rome that failed to utilize what they gained in Babylonia and Assyria. It is not as if there was nothing there for them to base a legitimacy upon.

 

[Hecatee]

Interesting maps have been built by the Orbis project at Stanford university, showing travelling time from Rome :

I clearly shows that the Empire's borders were set at a maximum of 35 to 42 days of travel to the capital, which can be understood as a maximum information spread speed limit to the size of the Empire.

The map showing transport costs is also quite telling :

Using those maps one may identify regions up for potential further roman integration, and they are not many...

 

[Atamolos]

First off, I'd agree that a Roman Western Europe could have been possible since the Romans have operated across the Rhine and half of Britain was already conquered, obvious POD would be the Julio-Claudian dynasty performing better, such as a long lived Drusus / Germanicus which butterflies the depressed Tiberius and Sejanus.

If only there was an active TL that fits that exact description...

On the economics, per Ward-Perkins research on the late Empire, there was a downside of the economical complexity of the Roman economy in the Western provinces, Italy having already been on a small downside trend and with the other western provinces starting to suffer the same effects in the mid 300's, adding Germania and the rest of the British Isles will not help in the eventual economical decline the Empire would suffer, neither were particularly rich or prosperous regions and would, in my view, would simply be weights on the Roman economy.

Can you link to these studies? I've read a few pieces of literature on the subject and they've portrayed views as varied as they are subjective. Some suggest that the so-called "economic decline" of the WRE was a function of rising labor costs; I've also heard it argued that the decline in the volume of bullion gives a false impression that there was an economic decline rather than a shift in the usage of capital, with a decreasing use of liquid capital in favor of physical commodities and hard assets accompanying a diversification of industries within a given area and a potential decline in large volumes of long-distance trade.

If you look at legion deployments, most of the legions were on the Danube, Rhine, and Syria. How many legions are actually in Greece, Macedonia, in Eastern Anatolia, in Italy, in Narbonensis Gaul, in Carthage, in Sicily, areas that were under centuries of Roman rule? And in Spain too?

Yes, but the primary difference here is that all of these areas were already heavily urbanized before their conquest by Rome. The exception to this rule is Spain, which by your own admission took centuries to fully pacify and even then remained under the watchful eye of a substantial garrison for at least a century. Britain has none of these administrative expediences, and unlike Spain, it is not accessible by road or by the Mediterranean, making it the single most expensive province to supply, without even considering the cost of maintaining the necessary garrison. The deployment of the legions in Britannia was not purely for frontier defense, and thus would not be necessarily reduced by eliminating the land border with Caledonia. The deployment of the legions as late as 410 reveals a different purpose for the garrison. This map shows how spread out the army camps were and this is a waste of resources if the purpose of the British garrison was primarily frontier defense rather than native pacification.

This would be in my opinion, the possible maximum expansion of the empire at a single moment. They may not keep it, but regardless, this could potentially be performed. A reminder, few polities in history have ever ruled a vast empire beyond fantastic rulers and periods of golden age. Only progressively did the Chinese states slowly integrate much of East Asia into their realm and sphere. Achaemenid Shahs did so only by way of the prior period which saw the subjugation of most of the vast region aside from Egypt and Anatolia.

I'd say you're right, except for Ireland may be pushing it a bit. It would be more probable for them to expand further up the Nile or further into Arabia than establish a strong enough port infrastructure on the West coast of Britain to support the invasion of Ireland.

Interesting maps have been built by the Orbis project at Stanford university, showing travelling time from Rome :

I clearly shows that the Empire's borders were set at a maximum of 35 to 42 days of travel to the capital, which can be understood as a maximum information spread speed limit to the size of the Empire.

The map showing transport costs is also quite telling :

Using those maps one may identify regions up for potential further roman integration, and they are not many...

Do these maps account for the Roman road system? Because I would imagine that makes a substantial difference. Also, the division of the Empire into a tetrarchy/diarchy or any analogous structure would reduce many of the effects of this distance (which is why Diocletian made those reforms IOTL)

 

[Hecatee]

If only there was an active TL that fits that exact description...

Do these maps account for the Roman road system? Because I would imagine that makes a substantial difference. Also, the division of the Empire into a tetrarchy/diarchy or any analogous structure would reduce many of the effects of this distance (which is why Diocletian made those reforms IOTL)

Yes they do take into account the Romans' roads and river/coastal infrastructures, so they already have almost maximum periode infrastructure to speed up movement. That's why I did include a type of telegraph in my timeline to make it possible to extend Rome's reach thanks to the new information spread rate.

 

[John7755 يوحنا]

@Atamolos

Hibernia might could be subjugated by way of the same method as Bospora, a collection of vassals who are slowly integrated and use Latin as an exchange tongue.

Nubia and Meroe might be possible as would Arabian coastal areas. Rome if it remained rulers in Babylonia, their power would at least spread into Gerhea and Dilmun to the south along the Arabian eastern coastline. However, there would probably more long-term effects in terms of language in Hibernia than in Nubia, where the culture would only be submissive to the notion of the Roman Phaoraonic claims/titles. Rome's propensity for rule is most exhibited in its ability to imitate and use the claims of the Bronze Age rulers of the Middle East. Rome failed to do this outside of Egypt and hence it was somewhat ephemeral in Babylonia. A great positive in my view, is if Rome took the title of 'King of the Universe,' King of the Four Corners and so forth, the titles that the Arsacid utilized in their rule over Babylonia, alongisde making inscriptions in Akkadian (as the Arsacid and Seleucids did).