Oh, empires are, cool, yes. But I'm going to monster post some more because, well, I like arguing (and I think you're understating the difficulties a little).
Ahh, but this additional expansion of the early Empire simply assumes that the major campaigns that it did undertake and you mention are more successful than they were OTL, something wholly plausible.
The right PoDs or butterflies (if we use a slightly earlier PoD like a surviving Caesar) make the conquest of western Germania as successful as the ones of Gallia and Britannia were OTL. The conquest of eastern Germania, Marcomannia, and Dacia would almost immediately follow after western Germania is pacified as the natural strategic completion of the former success.
As I said, I have no issue with granting the conquest of Western Germania. But "follow almost immediately... as the natural strategic completion" seems really strong. If nothing else, it will take two generations or so (time from Gaulic conquest to attempted conquest of Germania) for the natives to be sufficiently pacified for the legions to move on in any number (see, Teutoberg Wald) and enough provincial infrastructure to be built to support campaigns past the new frontier (and possibly longer, since Germania is if anything even worse country than pre-Roman Gaul).
Also, it's hard to say how far east the border could go: the Elbe-Ore-Carpathians line is good, and will garner the Romans Dacia and Bohemia as reasonable "natural strategic completions", but going further is just pushing out into worse and worse land, further and further from the centres of Roman supply, for negligible economic and strategic benefit.
Conquest of Britannia occurs on schedule a generation and half later, when the pacification of Germania and Dacia would be essentially done and military resources freed. The decision to annex Caledonia is relatively trivial to implement and may come as a plausible butterfly for various reasons, and it lays the ground to annex Hibernia as the natural extension of the previous conquest of whole Britannia. Same reasoning with Cimbria and Germania.
And again, the Romans had good reasons for not taking Caledonia IOTL. They knew Britannia was an island; they realised how much their strategic situation would improve is they could push the border off the end. But it simply ended up that drawing a line at the last marginally-civilisable* point was cheaper than trying to support the garrisons to hold down the end of the earth. Remember that Rome, like all empires, was built by the lowest bidder.
Hibernia is the same only worse, because it posed no strategic threat and is also
yet another ocean crossing from anywhere warm.
*in the "cities", not "culturedness", sense of the word.
As it concerns Nubia, Rome did get a major victory that would have allowed its annexation under Augustus, here is it simply assumed that the Empire simply chooses not to give the Nubians a lenient status quo peace. If Nubia is annexed, the later conquest of Axum after a generation comes as its natural extension.
I'll give you Nubia, but again, the words "natural extension" don't make the place next door any more economically or strategically viable.
And since the eastern border is much shorter and Germania-Dacia are already well underway to Romanization after three generations, and Britannia is already pacified, Trajan's empire has plenty of resources to stabilize the Zagros border and keep Armenia and Mesopotamia for good (and quite possibly make rump Iranian Parthia a client state).
The problem is that if the Zagros is a border it doesn't make a good frontier. The Romans - sensibly - tended to pick great honking massive geographical barriers for their borders, and the Zagros, coming from the east, isn't one. Armenia they might get but Mesopotamia is going to chew up a lot of legions - almost certianly more than were saved by the Rhine-Danube -> Elbe-Carpathia shortening. When *Hadrian comes around, Mesopotamia is still going to look like the best place for budget cuts.
As for Parthia - it typically took the Romans two wars* to properly clientize anyone, and that was with crushing victories in each war. The Romans never managed more than an "Honours go to" in its wars with Parthia, and increased aggressiveness doesn't change the fact that Ecbatana is a long, long way from Rome, and that horse archers tend to do well against heavy infantry.
*See, the other Italians, Carthage, Macedon, the Anatolian states, Egypt...
As you can see, this set of conquests does not assume that the early empire has to make any really extra major conquest effort, only that the ones they did are optimized.
I can count at least six efforts, totally absent IOTL, that are equivalent to any OTL imperial campaign: Bohemia, Nubia, the Elbe-Oder region (I assume that's Eastern Germania; Oder-Vistula is another campaign's worth, and well out of Roman reach anyways), Caledonia (if they bother); Hibernia (ditto), and the great deal of extar effort to hold the line of the Zagros.
The fine details about the apportionement of conquests between the various Emperors is wholly tentative and subject to butterflies, so something might well be conquered under Vespasian and not under a different emperor. See above for what I regard as the rough overall pace of conquest.
Well, butterflies would kill pretty much everybody after Tiberius (and for something like this I
strongly suggest you make Agrippa survive to be second emperor rather than Tiberius; Agrippa was in every way a better statesman, and you want as many good statesmen on top as possible).
OK, this is entirely reasonable, but conquering Persia and western Sarmatia is only an optional part of the scenario. However, TTL's Marcus Aurelius would wage his wars with the advantage of much more favourable borders on the Vistula-Dniester and Zagros, fully Romanized Germania, Dacia, and Britannia, and fully pacified Mesopotamia. It is entirely feasible that because of this, he may deem that the Empire can afford to turn his victories into some extra conquests. Of course, it is also entirely possible that he still enforces the status quo as IOTL, and whether annexing Parthia and western Sarmatia would be a net benefit or burden to the empire is open to discussion.
It
is Vistula-Dniester? Ungh, OK,
eight major campaigns (adding Oder-Vistula and Carpathians-Dniester).
More to the point, at this point the competent leaders are all going to be agreeing that the Empire
cannot afford more conquests. The roads and colonies are going to be getting hella few and far between out in darkest Poland; heavy plow or no heavy plow, it will be a while before they can support a Roman defensive frontier off the produce of Germania, and other supplies are a long way away. I would go as far as to say
campaigning on the far side of the Vistula would be essentially impossible, let alone making any attempt to conquer it. Parthia is less far away, conceptually if not as-the-crow-flies (Mesopotamian infrastructure would need severe upgrading, but at least it has agriculture and, eg,
farmers). But, again, Parthia is not going to be easy to take down and troops holding it are again going to be a long, long, long way from Rome and the Mediterranean.
Besides, garrisons on the Dvina and Dniepr could be easily supplied by river, those trade routes saw major traffic during the early Middle Ages, and by the time they would annex western Sarmatia, Germania and Dacia would be already fairly well developed. And what's this nonsense about Caledonia ? Rome can supply it by sea, crossing the Channel and coasting Britannia, quite nicely.
Dniepr, yes; but how about the Vistula? And I think you mean relatively well-developed
for Germania. There's not that many people here (the Germans haven't started multiplying yet, and even then they were always pretty underpopulated in Roman terms) and you need farmers from somewhere. (And don't say "Rome, of course"; Roman colonization outside the Mediterranean basin was tiny, and selling Varsavium is going to be even worse than selling northern Gaul.)
Caledonia wasn't supplied by sea because, for starters, Roman channel crossings went Calais-Dover and that was it.
Given Caesar's mindset and capabilities, and the extent of his OTL conquests, I think he would settle for nothing less than the annexation of Mesopotamia and Germania at the very least, and it would be well within his ability. He would likely harbor further ambitions about Persia, but it is quite possible that he would settle for making Persia a client, like he did with Egypt, and he may or may not succeed at it. Conquering Persia itself is much less feasible, at this stage.
He would go for Parthia, I'm sure, but managing to significantly dent Parthia is a tough task. Clientization is really hard, and I imagine would be rebuked at the first opportunity.
The Vistula-Carpathian-Dniester border would free up a lot of legions, Mesopotamia was the richest province of Parthia, a Persian state without it would be greately weakened, the Zagros outskirts make for an excellent natural border that Roman engineering could bulk nicely with a limes.
It's shorter, but you need to garrison the conquered provinces (the Romans had a legion-scale garrison in each of Galatia, Numidia, and Wales from the moment they conquered each to the moment they swapped out the legions for field armies in the 4th C; I have trouble believing there's nowhere in Magna Germania that would be as restive).
Annexing Mesopotamia gives a very rich and popolous province to the Empire, deprives Persia of it, and creates a border which is rather more defensible than the longer Euprathes-Syria border, a desert plain.
Rich, populous, and
non-Roman, not to mention eagerly coveted by the Parthians. And the Euphrates-Syria border was desert, yes - that's why it's defensible, the enemy is
on the other side of the desert. The Zagros puts
Roman frontier armies across a desert, practically on top of the enemy heartland (except downhill from it).
It is wholly reasonable to assume that the ownership of all that extra fallow land could spur the quick development of agricultural technology to exploit it in full. After all, the heavy plough and horse collar were fairly quickly developed IOTL, after Northern Europe was integrated in agricultural economy, in conditions much less optimal for steady technological progress than Rome at its heyday.
The Romans weren't actually that great at innovation; under the Empire, in particular, the phrase "sclerotic, slowly deteriorating mess" springs to mind. That said, the heavy plow and horse collar seem fairly reasonable.
This is entirely true, but there is more to it: the extra resources the Empire gains by owning Romanized Germania, Mesopotamia, Nubia, and Arabia Felix, the denial of same resources to barbarians and Persians, more defensible borders which make for a more indepth protection of the Empire's core, all those factors may easily mean that the Third and Fifth Century Crises are substantially diminished in their long-term consequences, they are no more the start of a death spiral, but a temporary crisis which Rome fully recovers from in due time, much like the Second Punic War or China's various dynastic crisis, or at the very most they may trigger the permanent division in WRE and ERE and nothing more. TTL turns Germanics and Arabs into extra resources from a big problem, and nerfs Persian empires. This is far, far from trivial about late antiquity crisis becoming a death spiral or not.
The Fifth Century crisis
was bad because of outside factors - but the Third Century crisis was all Rome. "More in depth protection of the Empire's core" - from who?
The problem was the Army. All putting the border further away is going to do is make it that much harder for central authority to recover it once the frontier armies start proclaiming their own Emperors. Conquered Mesopotamia above all is going to be an ever-loving
bitch to bring back in, but the other fringes are also going to be hard to take back.
Chinese civilization did not magically sprung fully formed into its late imperial borders at its birth. Its expansion was a millennia-long steady work of expansion, colonization and assimilation from its cradle alongside the Yellow River.
Yes... but the Han had China as China in the 1st C; all the famous breakups and reunifications of China after that happened in a subcontinent all one culture. China is a big, flat, tillable plain with no effect natural barriers whatsoever; Europe and the Mediterranean basin is a convoluted, forested, mountainous, watery, rocky mess. China's parts, even absent political union, tended to blend into each other; Europe's parts, even with political union, tended to go their own ways (viz, Alexander's empire, the third century crisis, the east/west division, the Diocletian reforms, the breakup of Rome.)
Give Rome some extra time and success, remove the Migrations, and the cultural imprint of Rome on Europe would become as indestructible as China had on its own mainland. The very existence and spread of Romance languages, and the huge grip the ideal of Rome had on later Europe, are proof to it.
I concede that the main long-term difference between Rome and China would be the deep-rooted dualism between Latin and Greek areas which is likely to last. But at the very most this can pave the way to recurrent or permanent division between West and East (China has always harbored a similar north-south economic/political dualism that never went to permanent division). Give Rome some extra optimal headstart, Latin-Greek culture would eventually and totally affirm in Romasphere, and the ideal of imperial unity would become as unshakable in its ruling and middle classes as it was in China.
They didn't manage the Greeks; I doubt they could manage Mesopotamia; and (as for success in assimilating northern Europe) Britain went post-apocalyptic wasteland within a generation of the Army leaving. Roman culture was never that entrenched in the east, where there were already lots of other cultures; and the west it just never rooted more than superficially at all. The moment the Romans stopped raising taxes from cities to pay farmers to feed armies, the taxes, cities, farms, and armies all withered up and blew away. That was with 400 years; how much longer do you want?
Again, a more successful Empire could well evolve into a WRE/ERE division, because of underlying dualism, but this is far, far different from the OTL fragmentation. Romanization of the West was well underway (see Romance languages) and it was only reversed because the West was carved up into tribal kingdoms by Germanic migrations. ITTL those Germanic peoples would be as Romanized as the inhabitants of Gallia and Iberia, residual unassimilated barbarian tribes from Scandinavia and Sarmatia and the steppe nomads simply would not have the manpower to implement the cultural and political substitution of Roman Europe that Germanics did OTL, even assuming the Huns stage a successful conquest of Rome or the WRE/ERE (a very questionable assumption with this Empire), at the very most it would be a temporary dynastic takeover that would not harm the lasting cultural and political integrity of the Romasphere.
Why would the barbarians need manpower? They didn't have manpower; they had the stirrup and an enemy with arteries harder than rock. It was easier to hire mercenaries than support an army (see the lowest bidder comment above) and the late Empire had a culture where success = assassination. Given those circumstances, why
wouldn't barbarians be able to start swing off pieces?
Your statement about the archeology of Roman Britannia is hilarious, and anyway ITTL Saxons would be as Romans as the Britons themselves.
I'm happy you find it so but it's true. Central authority, the cities, even the quality and distribution of material goods dropped off like the Romans had never visited the island.
About Roman steam engine, I fully agree that the Empire did lack the technological basis to make it work in its OTL span, but this is not nowhere necessary to its survival. An empire that makes a lasting recovery from its late antiquity crisis shall butterfly away the Dark Ages, and steadily proceed to the technological advances of OTL Islamic Golden Age and European High Middle Ages, either by autonomus development or exchange with China. That's more than enough to remove most of the technological weak points that the Empire had.
A clichè with very little basis in fact. The fact is that political and economic collapse of the late empire was not exactly optimal for progress.
I'm not quite sure what to make of this last paragraph - Rome wasn't stagnant and anyways the stagnation was the fault of all the instability? But it was stagnant.
Ever heard of Vinland ? When Rome discovers America, either by itself, or riding on the coattails of Norse explorers, it shall be by following the Iceland-Greenland-North America route, I agree that a replication of the Columbus voyage is terribly unlikely.
Yeah, I thought of longships later and kinda edited that into the post after the fact.
But Vinland was really, really,
really marginal. Newfoundland and Labrador are not remotely inviting prospects, and certainly not if you have to cross the North Atlantic to get there.
True, at the most they have Persia as a block, assuming they don't assimilate it or make it a client. Moreover, since they fully control the Red Sea route all the way to Yemen and Ethiopia, they have the incentive to maintain and improve the Suez canal, develop a decent ocean-going technology, and go by the India-Indonesia route.
However, once they do develop a decent ocean-going technology, they are bound to start major-scale exploring. Mastering the Indian ocean route to China and to a lesser degree circumnavigation of Africa shall be their top prorities, but since they hold the British Isles, sooner or later they are going to explore the Iceland-Greenland-Vinland route as well. The huge agricultural potential of North America cannot escape the notice of Roman explorers, and Native Americans are a trivial concern for Renaissance Roman settlers and legions.
You need rather different ships to cross the Indian and Atlantic oceans. The Arabian Sea is relatively nice, as these things go, and has incredibly dependable winds; the Atlantic is not nice, is wider and has no friendly shores around it. (also: why circumnavigate Africa when you're already on the Red Sea? Portugal only bothered because the Ottomans were in the way, the Romans sure have no need to.)
However, I easily concede that if Rome is strong and technologically advanced enough to undertake a major exploration and colonization of the Americas, its first priority shall be colonization and assimilation of Sarmatia, Persia, and Central Asia, exploration/trade with India, Indonesia, and China, and quite possibly they can eventually envisage conquest of India, esp. if Persia was assimilated long ago, but it is also possible they are happy with making India a client.
Rome will always have bigger priorities than America, definitely, but
conquest of India? Clientizing Persia is at the edge of possibility at the height(s) of Roman power; conquering your way to the Ganges is... just...
If the survival and expansion of Rome butterflies Indian unity into existence, it may or may not have the resources to withstand Roman penetration. A disunited India is rather unlikely to withstand Roman colonization, even if it is only done by a WRE or ERE, if it stays independent it is most likely because competition with other roman half or China stalemates expansion.
A disunited India will do whatever the heck it likes, because it's 6000 km away.
Persia proper or a united Rus (say Rome's survival deflects Norse expansion almost entirely into Sarmatia, and they create a sturdier Rus than OTL) have far less chances, they absolutely would not have the resources to withstand expansion by a united Rome with Exploration Age technology. A Renaissance WRE would very very likely swallow a united Rus but it is not a given, a disunited Rus has no chance. Ditto for Persia and Renaissance ERE.
Rome's made it to the Renaissance now... but that doesn't make the empire smaller (you need railroads and steamships to do that meaningfully) and it's still got structural issues until I'm convinced otherwise.
Norse colonies in the Americas are quite possible, a Norse America is feasible but only if Rome is too busy expanding in the East to bother about the New World for several centuries.
You ever been to northern Newfoundland?
Not necessarily. This empire has the vast majority of its legions concentrated on two relatively short borders in comparison to OTL, Vistula-Dniester and Zagros. It is quite likely that during dynastic crises, the Sarmatian and Persian legions are going to come up with one pretedent each relatively quickly, either by agreement or battle. Besides that, one extra pretendent may arise in Rome from the Senate or the Pretorians (if these bodies disgree, one victorious pretendent shall arise among them very quickly for obvious reasons).
Most of the legions are in Sarmatia and Mesopotamia (although that's going to make the Mesopotamian Empire essentially unstoppable, as mentioned before) but do you know what the cutoff for putting an Emperor on the throne was in the Third Century?
One legion. There
will be usurpers from everywhere, and the big frontier armies (even further away than in the 270s!)
are going to be hard-to-impossible to put down if/when they revolt.
It means that at the worst, dynastic crises shall see 2-3 pretendents, civil wars shall be intense but quickly settled and the recurring tendence of the empire shall be to split into western and eastern halves. This all indicates that civil wars are going to be no more destructive than OTL and shall bring west-east dualism into fore, even more so than OTL.
Most of the legions were on the Rhine, Danube, or Syria IOTL, but even though most of the successful candidates came from those three it didn't make the 3rd C any less of a confused, impenetrable muddle. Reducing the big borders to two won't pull the one legion, or its general, from Numidia.
A lasting dynastic cycle of west-east dualism, much like the Chinese north-south one, is the most likely natural outcome. Because Rome dualism was fueled by a cultural divide that China lacked, at some point this division may or may not become permanent, but that's the only major difference.
Except the geography is different, as I said: China is one big place and Europe is a bunch of distinct and rather different little zones. In China, different states never had any real cultural divergence to speak of; in Europe, the successor states (all identically Roman provinces run by Germans) started becoming culturally different the moment they stopped being stapled to each other.
I agree that is questionable whether Rome would keep Persia or Sarmatia after a major victory on either front, but conquering either looks quite feasible if the other border is quiet.
That paragraph seems to contradict itself.
Certainly lasting conquest or vassallization of Persia seems within this Empire's grasp. I agree that a complete cutural assimilation of Persia is not that more likely than a solution to Latin/Greek dualism, but neither is Persia remaining more unaffected by long-term Roman domination than OTL Arab domination.
It's worth noting Persia spent something like two hundred years under Arab domination, and at the end of it the only real difference was they had converted to Islam. And even then the Caliphate was based out of Mesopotamia next door, not the Central Mediterranean a long way away.
A vassal or annexed Persia would turn the Latin-Greek dualism into a three-way Latin-Greek-Persian divide. However the limited dimensions and resources of Persia in comparison to the WRE and ERE would not make it any substiantially worse than the original dualism, and most likely this could pave the way to the permanent breakup of Persia (again, until the roman successor empires master Renaissance technology, then it0s a wholly new game).
A vassal Persia wouldn't last and a conquered Persia would revolt even faster, as I keep saying. It's just too bloody far away.
Western Sarmatia was a largely empty place with a scattering of Finnish, Germanic, Slavic, and Iranian tribes. As you indeed pointed out, like Persia, defending the place would add significant logistical burdens to the Empire, but militarily the conquest itself would not be that much difficult.
And therein lies the problem: why would they take it, and why would they keep it?
Just like Germania, if Rome does hold the place for long, it can be gradually developed by a mix of Roman colonization and native settlement, until it becomes something much like the Middle Age equivalent, at such a point it would become a net asset and not a burden to the Empire, or at least significantly contribute to paying for its own defense.
You seem to be significantly overestimating Roman ability to colonise places. In the year 450, Rome had two breadbaskets, Africa and Egypt, which
not coincidentally were its two breadbaskets in 1CE.
I agree that demographic, economic, and logistical conditions (if the heavy plough and horse collar are available) are such that Rome would develop Germania much quicker than Sarmatia. By the time that the crisis of late Antiquity shows up, Germania would be essentially indistinguishable from the rest of Roman Europe, while Sarmatia would be at best an half-way patchwork. But the same model could be successfully applied.
Well, Germania would be indistinguishable from Northern Gaul or Britannia. You could always distinguish them from the Mediterranean provinces (which were largely distinguishable from each other).
And a strong Rome could crush their uprising in an annexed or client Persia just like it did so for many other nationalist revolts. Ask the Jew Zealots.
The *Sassanids only have a good chance if their rise is timed with a Roman dynstic crisis, and even so, it is questionable whether they can withstand the offensive comeback from a recovered Rome.
The Jews were outnumbered by the Persians to a couple of orders of magnitude, and also live on the edge of the big legion-transshipment hub known as the Mediterranean. The Persians live across the sea, across a desert, and up some mountains.
Very true. OTOH, the Zagros are a much more defensible border than the Euphrates, they weaken Persian Empires a lot. The advantages of conquering or vassallizing Persia are twofold: it removes the last developed rival on Rome's borders, and it gives Rome full control on trade routes to India and China. They are far from trivial, and need to be weighted against the more extended borders on the east, as the respective benefits and drawbacks of annexing or vassallizing Persia.
It would still cost too much. The silk road was important but the amount of trade the Persians blocked, versus the cost of conquering, garrisoning, and suppressing the entire Iranian Plateau, makes it a no-brainer.
True but in the long term, conquered Sarmatia shall be colonized and developed to Middle Age levels, and the lack of local support shall no longer be an issue.
Who gives a damn about long term? Lowest bidder, remember? (And see: the Emperor Hadrian. You just need one of him.)
Neither the Huns nor the Sarmatian tribes in the 4th-5th century have nowhere near the population basis necessary to carve up major bits of the Empire into lasting tribal states, even if they manage to stage a strategic breakout of the Sarmatian limes, which is only likely to succeed if Rome is in the grips of a dynastic crisis. At worst, whatever they grasp shall be almost surely reconquered by Rome in the dynastic collapse the Hun empire suffers after *Attila's death. Without the Germanic Migrations, the Huns don't have the population bulk to make lasting damage to the Romasphere, like they did not to the Chinasphere.
The words "dynastic crisis" seem to be coming up a lot too. Rome didn't have dynastic crises: it had coups, which were followed by more coups, followed by a strongman, followed by the strongman's idiot offspring, followed by coups. The Roman executive was nowheres near as stable as you seen to be assuming.
No strain whatever, since those gains provide better borders and extra resources to Rome, and deny them to its enemies.[/quote]
There's no enemy that needs to be denied Arabia Felix or Nubia, and the resources provided by Germania are going to be pretty negligible for a while (heavy plow + horse collar + population growth = not six weeks after the conquest).
And the problem with supporting garrisons and colonies in southern Sweden with naval shipping from Britannia, northern Gallia, Cimbria, and Germania would be ???
It is true that if it happens at all, Rome would undertake the conquest of Scandinavia only if and when western Sarmatia is conquered and fairly settled, and the Huns are repelled, so substantially beyond late Antiquity timespan. Quite likely, if it happens at all, it is as a military reaction to Norse expansion. By the time Rome seriously plans about expanding into Scandinavia, western-central Europe shall be as Roman as Italy.
Again, the Romans were good at sailing the Med. The Channel Fleet lived in the Straight of Dover; they practically never went further up the island than London, let alone to Sweden.
(And again - colonies in Sweden
why? By the time they're a threat you can no longer just jaunt up and down their coasts.)
Just like Scandinavia, eastern Sarmatia could and would be conquered only if and when western Sarmatia is conquered and fairly developed, so that it can support further expansion. It would happen well beyond the span of late antiquity.
Again - if the Empire makes it.
OTL + Germania, Dacia, Nubia, Mesopotamia, Arabia Felix may be easily done by the 1st century empire with the right PoDs and butterflies, but not much more than that.
Yes, I will buy that.
Shaky conquest or vassallization of Persia in early or late 2nd century may or may not happen, just like shaky conquest of western Sarmatia in late 2nd century. Temporary vassallization of Persia in 1st Century only if Caesar is around to pull it with one of his military miracles, and then again a mess to maintain until Germania and Britannia are basically done.
Again agreed, although I'd say Persia and Sarmatia are never going to come under Roman control, for the simple reason they're too far away, and too strong and too poor respectively.
A century of overall defensive consolidation and two centuries of development would make western-central Europe basically Romanized, four centuries of development would make it fully so. Its development creates the basic demographic and economic groundwork Rome would need to entrench assimilation of western Sarmatia and/or Persia in earnest.
You've just counted five centuries (four if I'm reading your grammar wrong). How are you holding the political system together this long?
Depending on butterflies, the crises of the 3th and 5th centuries may or may not hit the Empire with varying severity (some components, like the Huns and plagues, are much less liable to TTL's butterflies) but in all likelihood the lasting damage to TTL Empire is substantially lessened (the Empire has more resources and opportunistic external enemies have less).
I will say it again: the Third Century Crisis was the logical outcome of the deep-seated and critical flaws in the Roman / Imperial structure. It had almost nothing to do with outside forces. It
will hit unless you come up with some major improvements to Rome's politics, and unless you can come up with a solution that keeps things where they were then it
will do lasting damage (the post-Crisis Empire was stable but it came out a naked military dictatorship far less economically or militarily sound than the pre-Crisis Empire had been).
Depending on their severity, the timeframe varies for the Empire to undertake the assimilation of Persia and/or western Sarmatia with really good chances of success, but at worst it would show up in the 4th or 6th century. If such expansion is done, the Empire is going to need another centuries-long cycle of development and consolidation before it can expand any further. For various reasons western Sarmatia and Persia are rather more complex to assimilate than previous conquests, so any further expansion is utterly unlikely before the start of the Norse Age and technological development to High-Late Middle Age levels or so.
Assuming the Empire survives.
Big letters here:
Rome's problem was never its external enemies, it was the decay of its political structure.
True, but see above about Persia. Even making Persia a client would make a substantial difference, and if they forsake or delay Sarmatian expansion, annexing and holding Persia proper is not that much difficult without the Germanic headache, if the bulk of Roman might is focused on it, Alexander and the early Caliphs conquered Persia fairly easy and their successors kept it for centuries, militarly and technologically they were no way superior to the Romans, and this Roman Empire fully controls the Red Sea, so it can supply garrisons in Persia by circumnavigation of the Arabian peninsula too.
The Persians are not the Germans, they're not even the Greeks; Parthian horse archers
are better than the Romans and if the Romans can win it will be by logistical bludgeoning, which will be hell past the Zagros.
Persia proper is not so vast nor populated than suppressing separatist revolts would be an headache task for a strong Rome (although I concede that Persia would tend to break away in any serious dynastic crisis). The main drawback of conquering (or vassallizing) Persia is that it give a rather worse border to defend against Central Asian nomads than the Zagros (even if the nomads would be a much less serious enemy than an hostile Persia). Anyway, even if Rome does conquer or vassallize Persia, its long-term control would perforce be limited to the Iranian plateau itself, any attempt to expand in the AfPak area would be a foolish venture, an embarassing failure, and quickly be abandoned.
And, may I remind you, dynastic crises in Rome happened at the drop of a hat. Persia will be in the empire for a half-century, tops. And, uh, Rome lived next to Persia for a millennia - nobody lies next to central Asian nomads happily.
I'd argue any attempt to expand onto the Iranian Plateau would be a foolish venture, an embarassing failure, and quickly be abandoned, but we've been over this.
This is already far far too long; I won't argue the religion, since it's not the world's most objective subject, and I'll admit I know more about Roman politics and economics than religion.
Unless the Empire is facing the Huns on the other border, or is in the grip of a major dynastic crisis or temporary split, Persian revolts shall fare no better than Judean ones. The Sarmatian border is only going to be a major concern for a strong Empire when major tribal concentrations of steppe nomads show up, which they can only do every few centuries. The bulk of the Empire's military might can be easily refocused to suppress Persian revolts, there is not much else legions can be used besides guarding the Sramtian borders (and fighting civil wars), the Iranian plateau is not that much remote nor large nor populated.
Civil wars give such a revolt a chance to succeed, but what is lost during a dynastic crisis can always be reconquered when the Empire recovers, only if the WRE and ERE split permanently Persia gets a really good chance to split off for good as well.
It is that remote and the Romans did not just throw legions around. Moving legions in any kind of reasonable timescale was pretty difficult; moving them around outside the Med (where, coincidentally Judea is located) is a task measured in years. By the time they get there they'll have a reconquest on their hands.
Which ones ? Germanics from Central Europe (and later Arabs) were the vast majority of them OTL. Here, they are loyal Roman citizens and bulk up the legions. Residual unassimilated Germanics and Slavs in Scandinavia and Sarmatia are a rather pitiful smattering in the timespan of late antiquity when barbarian migrations could build on dynastic crisis and steppe-nomad recurring expansion and make OTL lasting damage. They don't have the manpower. When they shall have built it up and steppe nomads shall show up again in 8th-9th century, the Empire (or its WRE/ERE successors) shall be so much demographically, economically, and culturally developed and integrated to make itself as invulnerable to lasting breakup as China.
OK then, Berbers, Arabs, whoever comes down from the steppes through Persia, whoever comes across the steppes through Sarmatia.
Sorry, this is an entirely arbitrary and unimaginative copy and paste of OTL on a TL with wholly different foundations.
Because... Iberia is not a geographical unit on its own? Africa Is not surrounded by desert? Britian is not kinda separated from other parts of Europe?
OTL tribal kingdoms and later nation-states only could arise because the invading Germanic tribes put them into shape. Here, barbarians are far too few to implement that kind of lasting change.
No, they arose because the
moment the Emperors in Ravenna stopped trying to exert authority outside of Italy, there was nothing to keep them from drifting apart. It doesn't matter if it's a German Warlord or a Roman General leading it, once Iberia stops looking at Rome for what Romanness is, it will start to come up with its own ideas thereon.
Both OTL patterns of splintering in the Roman Empire before the Migrations and the structure of TTL Empire indicate that during dynastic crises the Empire tends to divide into western and eastern halves, or at the very worst a Western-Central European "Carolingian" third, a "Byzantine" Greek-Middle Eastern third, the Italian-and-Mediterranean-stuff third that is too fragile and composite for lasting separation, plus the optional Persian shard.
by which you mean the Gallic and Palmyrene Empires, I guess. I'm seeing four there: Gaul, remnant West, remnant East, Palmyra. Oh, plus Britian, which kept producing its own emperors endlessly. And the Balkans, which had their own series of Danubian Emperors, and Illyrian Emperors. And Africa. And Iberia, which kinda flipped back and forth between Gaul, Rome, and autonomy, and -
By Jove, it's the Successor States!
Worse fragmentation does not tend to manifest without major barbarian settlement, and the more the Empire lasts, the more unlikely it becomes, since the cultural and economic pulls to unity become stronger and the demographic base of the Empire grows, making barbarian inroads pitiful in comparison and easily repelled or assimilated.
Sorry, without all those unassimilated Germanic tribes teeming beyond the Rhine and Danube, the nation-state fragmentation of Europe is doomed to stillbirth, oh joy.
And they shall fare no better than with China. The legions kick their butts most of the time, every few centuries dynastic crises give them the window to set up a dynasty of their own, but that's all. The smattering of steppe nomad conquerors is culturally and demographically swallowed in the Roman sea. Eventually, Rome (or its WRE/ERE successors) pull themselves together, overthrow the foreign rulers, and endure.
I think this whole argument can be summed up as: read a good book on the Crisis of the Third Century. Rome was never as unified as you seem to think, and never as stable.
Look, I like Rome too. It's awesome. But it wasn't a conquest-a-generation military machine (except briefly in the second century BCE, and that was coupled with a god-awful series of internal crises from the strain) and it had a great deal of deep structural problems. Your arguments tend a lot towards why its enemies are weak and can't beat it. Rome's enemies
were weak, relatively speaking. But they were never its problem! The Carthaginians under Hannibal were far, far stronger than any of the German tribes - then all the German tribes combined, probably - but Rome conquered Carthage and folded like a deck of cards to the Germans, because in the intervening six centuries its arteries had hardened so much it couldn't deal anymore.
That is the problem you have to deal with, not Alaric, not Atilla, and not Ardashir. I keep harping on this, at incredible length, because I don't see that you're getting it.
Sorry to rant.
I'll try to be shorter next time.
Zyzzyva