How big of a threat would a united Germania be to Rome?

[Flavius Iulius Maiorianus]

Let's say for the sake of the scenario that the Western Roman Empire endures into the middle ages with its border on the Rhine. Maybe the Gothic migrations are handled better, and they either don't revolt or are defeated at Adrianople and scattered around the empire. My question is, if all of Germania between the Rhine, Danube, and Oder-Neisse line is united, how big of a threat to the Roman Empire would it be?

Obviously no answer can be given without some perameters. Let's say this German kingdom is a centralized monarchy similar to OTL's France. The monarch's power is not totally absolute, but it's getting there. Its western border is on the Rhine river, and it includes the bit of the Netherlands north of the Rhine. Its northern border is the northern border of OTL's German Confederation. Its southern border is on the Danube, and its eastern border is the same as OTL Germany. My question is, how big of a threat would it be to the Roman Empire during the medieval period? What about the early modern period? And what about the industrial revolution, assuming it still happens in a roughly similar timeframe as OTL?

 

[Alex1guy]

If you have the Western Empire survive with its borders on the Rhine, for like.... Another 500 to 1500 years, who knows how strong or weak the Empire is. You're just completely making things up at that point, nothing about Europe is recognizable.

 

[anotherlurker]

Let's say this German kingdom is a centralized monarchy similar to OTL's France.

Less of a threat than decentralized, rural barbarians because such a state can be destroyed and conquered in 2 or 3 battles, which is what the Romans did to all the post Alexander states in the East or Carthage. Barbarians are much harder to deal with, what you gonna do, conquer some bushes and trees?

 

[Nuraghe]

Less of a threat than decentralized, rural barbarians because such a state can be destroyed and conquered in 2 or 3 battles, which is what the Romans did to all the post Alexander states in the East or Carthage. Barbarians are much harder to deal with, what you gonna do, conquer some bushes and trees?

quite agree with you, instilling a centralized state on your border, certainly worsens the situation in case of war ( because it is more prepared, compared to a loose confederation of peoples, and considering the German population in the Middle Ages, they would be truly important numbers on a military level, perhaps equal to the Sassanid armies ) but in case of victory, just march on the capital to ask for a yield, furthermore we must not forget how trade and diplomacy also gain considerably from a similar development ( if we consider that Rome at its peak had to deal with at least 200 Germanic entities, all different from each other, perpetually warlike, with complex rules and traditions and who lived in an area highly unsuitable for the fighting style preferred by the legions, it is incredible that the local officials did not go mad from the chaos with which they had to work daily, in managing the Roman territories and dealing with diplomatic affairs in the "Barbaricum" to keep pacified the region and control trade flows ) having only one neighbor simplifies this whole process extremely, furthermore it allows to encourage the urban development of the Trans-Rhine region, which in case of future conquest, allows an easier application of the Roman government, because there are already some good starting bases that can be exploited, compared to having to build one from scratch as in Otl, to conclude the presence of another strong regional player in Europe will mean that Rome will necessarily have to engage in a sort of arms race to remain superior to it, which could potentially be good for the Empire ( I'm imagining a situation similar to Otl competition between European states in the late Middle Ages - early Modern Age, which forced the main actors to carry out processes of state centralization ( at various levels, combined with various cultural effects that resulted from it ) to keep up with their opponents )

 

[Evil Crusader]

Let's say this German kingdom is a centralized monarchy similar to OTL's France. The monarch's power is not totally absolute, but it's getting there. Its western border is on the Rhine river, and it includes the bit of the Netherlands north of the Rhine. Its northern border is the northern border of OTL's German Confederation. Its southern border is on the Danube, and its eastern border is the same as OTL Germany. My question is, how big of a threat would it be to the Roman Empire during the medieval period?

It would probably be fairly close to the Sasanids: another peer competitor to be taken seriously. After all, if they attained such a level of centralisation and survived to attain it, they're likely to adopt a significant portion of the Roman customs and technologies that gave them the edge, and much like the Iranians, their best expansion vector is towards Roman lands rather than the cold steppes where horse archers dwell. They're likely to adopt Arian Christianity, if they don't reform one of their native cults, too.

What about the early modern period? And what about the industrial revolution, assuming it still happens in a roughly similar timeframe as OTL?

Something eventually gives, but ATL will be very different than OTL. To even think of this question is to undertake historical butterfly genocide, and the answer likely is authorial (whatever you want it to be for storytelling purposes) because of the many, many ways the pieces can be made to fit together.

 

[Alex1guy]

It would probably be fairly close to the Sasanids: another peer competitor to be taken seriously. After all, if they attained such a level of centralisation and survived to attain it, they're likely to adopt a significant portion of the Roman customs and technologies that gave them the edge, and much like the Iranians, their best expansion vector is towards Roman lands rather than the cold steppes where horse archers dwell. They're likely to adopt Arian Christianity, if they don't reform one of their native cults, too.

Something eventually gives, but ATL will be very different than OTL. To even think of this question is to undertake historical butterfly genocide, and the answer likely is authorial (whatever you want it to be for storytelling purposes) because of the many, many ways the pieces can be made to fit together.

Yeah if you're writing a narrative you get to be in full control over a time span that long. Authors choice essentially

 

[Arkenfolm]

The Holy Roman Empire was a major powerhouse in medieval Europe readily capable of exercising its influence in Italy, so the answer is a similar state would be an incredible threat. Especially since they have very secure borders.

Now IMO such an absolute monarchy is somewhat improbable. Germanic chiefs did not like other chiefs interfering in their affairs, nor did Germanic princes of medieval times. Germanic kings were always elected by powerful followers, and this system isn't one which encourages centralisation or anything approaching absolutism. It's also difficult since Germany is mostly a bunch of forests during the 1st millennium AD and even then there's still ample land to settle in the east--not an economic situation that supports centralisation. I think such a system would need an alternate pathway to emerge, like a Hunno-Germanic empire where Attila's descendants have some sort of godlike status like the Borjigins attained or through religion like the king being the high priest of Tiwaz/whoever (i.e. "pontifex maximus") or a caesaropapist Arian church.

Less of a threat than decentralized, rural barbarians because such a state can be destroyed and conquered in 2 or 3 battles, which is what the Romans did to all the post Alexander states in the East or Carthage. Barbarians are much harder to deal with, what you gonna do, conquer some bushes and trees?

Why would that be the case? Such a Germanic state is likely to be incredibly decentralised and indeed, mostly bushes and trees. The only difference is that by necessity it would have slightly higher levels of urbanisation for collecting taxes and redistributing wealth as well as keeping potentially rebellious chiefs in line. Invading a mostly rural and decentralised state with few population centers to capture and exercise control over is a challenge. Caesar won in Gaul, but it was a complex campaign and he was a fantastic general. A campaign with even worse logistics for the Romans (have to cross the Rhine or the Alps) and a less capable general could easily see a coalition of Germanic tribes united into a single kingdom (who by OP's scenario would have fewer defectors to the Romans than the Gaulish tribes did) smash the Romans either as they try and lay siege to these forts or in an ambush situation like Arminius's victory.

 

[Flavius Iulius Maiorianus]

I was intending to look at it more from a natural resources perspective.

The Holy Roman Empire was a major powerhouse in medieval Europe readily capable of exercising its influence in Italy, so the answer is a similar state would be an incredible threat. Especially since they have very secure borders.

Now IMO such an absolute monarchy is somewhat improbable. Germanic chiefs did not like other chiefs interfering in their affairs, nor did Germanic princes of medieval times. Germanic kings were always elected by powerful followers, and this system isn't one which encourages centralisation or anything approaching absolutism. It's also difficult since Germany is mostly a bunch of forests during the 1st millennium AD and even then there's still ample land to settle in the east--not an economic situation that supports centralisation. I think such a system would need an alternate pathway to emerge, like a Hunno-Germanic empire where Attila's descendants have some sort of godlike status like the Borjigins attained or through religion like the king being the high priest of Tiwaz/whoever (i.e. "pontifex maximus") or a caesaropapist Arian church.

Why would that be the case? Such a Germanic state is likely to be incredibly decentralised and indeed, mostly bushes and trees. The only difference is that by necessity it would have slightly higher levels of urbanisation for collecting taxes and redistributing wealth as well as keeping potentially rebellious chiefs in line. Invading a mostly rural and decentralised state with few population centers to capture and exercise control over is a challenge. Caesar won in Gaul, but it was a complex campaign and he was a fantastic general. A campaign with even worse logistics for the Romans (have to cross the Rhine or the Alps) and a less capable general could easily see a coalition of Germanic tribes united into a single kingdom (who by OP's scenario would have fewer defectors to the Romans than the Gaulish tribes did) smash the Romans either as they try and lay siege to these forts or in an ambush situation like Arminius's victory.

The difference between the HRE and this alternate German kingdom is that the HRE had the incredibly rich low countries and Burgundy, as well as the lands south of the Danube. Since this kingdom lacks all of that, would it really be able to expand east the way OTL'S HRE did?

 

[Jürgen]

Honestly question like this depend on technology. Until the invention of the heavy plough the North European plains was thinly populated. The wooden ard plough had a hard time dealing with clay rich soil, so most of the good land was mostly left to grazing and forest, while agriculture was mostly on worse sandy soil. It’s why the region around Denmark was one of the more densely populated regions until the high Middle Ages thanks to the access to rich fishing grounds, and why we saw expansion of the different Germanic people from it.

But the moment the heavy plough have been invented suddenly Germania can feed massive populations, and it’s a significant bigger threat.

 

[Arkenfolm]

I was intending to look at it more from a natural resources perspective.

Good agricultural land is uncommon before the heavy plough but there is sufficient grazing land and absolutely no shortage of timber. Germany also had the Amber Road and early tin mining in the Ore Mountains.

The difference between the HRE and this alternate German kingdom is that the HRE had the incredibly rich low countries and Burgundy, as well as the lands south of the Danube. Since this kingdom lacks all of that, would it really be able to expand east the way OTL'S HRE did?

The Frankish Empire did at the end of Late Antiquity. There's nothing in the east beside the East Germanic tribes and later on the Slavs and maybe Sarmatians. And of course invaders from the steppe like the Huns, Avars, and Magyars, but the former is a really good candidate to unite most/all of Germany.

In Antiquity, it's also clear that the vicinity of the Rhine and Danube was fairly wealthy by Germanic standards, no doubt due to proximity to the Roman Empire. A united Germany would probably be more socially complex earlier and have more urban centers (even if they're only small towns of a few hundred people) and internal trade (since the king wants to collect tribute from his subordinate chiefs and make travel easier for himself, his government agents, and his armies).

Honestly question like this depend on technology. Until the invention of the heavy plough the North European plains was thinly populated. The wooden ard plough had a hard time dealing with clay rich soil, so most of the good land was mostly left to grazing and forest, while agriculture was mostly on worse sandy soil. It’s why the region around Denmark was one of the more densely populated regions until the high Middle Ages thanks to the access to rich fishing grounds, and why we saw expansion of the different Germanic people from it.

The heavy plough seems to have already existed in Late Antiquity Europe, so a united Germanic state could maybe adopt it in the early 5th century or so (maybe by captured peasants obtained from raiding Gaul).

Even before then, I'm not sure forests and pastoralism would necessarily deter the formation of a united entity on Rome's northern border anymore than it didn't deter the formation of such entities on China's northern border. Or for a much later example, the Iroquois Confederacy (although they were farmers and not pastoralists, they didn't have the complex agricultural economy typical Old World states had). The Iroquois weren't a large state, but in their earlier days they also didn't have access to horses like Germanic elites did for sending messages.

 

[anotherlurker]

Why would that be the case? Such a Germanic state is likely to be incredibly decentralised and indeed, mostly bushes and trees. The only difference is that by necessity it would have slightly higher levels of urbanisation for collecting taxes and redistributing wealth as well as keeping potentially rebellious chiefs in line. Invading a mostly rural and decentralised state with few population centers to capture and exercise control over is a challenge. Caesar won in Gaul, but it was a complex campaign and he was a fantastic general. A campaign with even worse logistics for the Romans (have to cross the Rhine or the Alps) and a less capable general could easily see a coalition of Germanic tribes united into a single kingdom (who by OP's scenario would have fewer defectors to the Romans than the Gaulish tribes did) smash the Romans either as they try and lay siege to these forts or in an ambush situation like Arminius's victory.

Germany in Roman times had around 4 million people on an area far bigger than todays Germany. An agricultural state can't put 30 % of its population under arms like barbarians can, so unless they multiply and cut down their defensive trees and bushes to make room for more fiels you'll have a far weaker politiy than the OTL patchwork of barbarian clans.
Caesar is a fantastic general mostly because he tells you he's a fantastic general, people shouldn't put too much weight in the words of a liar as notorious as old Julius. There's plenty of other capable people, Tiberius, Germanicus and Drusus had no problems rampaging through Germania.

 

[Hearkener]

The Holy Roman Empire was a major powerhouse in medieval Europe readily capable of exercising its influence in Italy, so the answer is a similar state would be an incredible threat. Especially since they have very secure borders.

The only reason the HRE was able to do that was because they were the biggest kid on the block. Against a Roman Empire which spans from the First Cataract to the great North Sea, they'd be dwarfed. Germania would never be a peer power to Rome, because unlike Persia, they don't have the mass. In a total war scenario, it would be a repeat of the Punic Wars; no matter how well the Germanics fight, they just don't have the manpower (or the money), and they'd inevitably be crushed by Rome's superiority in both of those areas.

I might add that Germania doesn't have as secure borders as you might think: the Rhine is not really difficult to cross with Roman engineering, and aside from the Black Forest and the swampy, (Malaria-ridden!) reaches of the middle Rhine, western Germany is well suited to campaigning with large armies. The Alps are the only major obstacle, really.

 

[ike225]

So Rome never fell and remains the same size as during the 4th century?
Rome is the powerhouse in europe since is never had to deal with the collapse of the empire and the loss of population, knowledge, collapse of cities,…
Rome is a faction with massive standing armies

Also how centralized is the Germanic kingdom? You say like France, but France of what era?

 

[Flavius Iulius Maiorianus]

So Rome never fell and remains the same size as during the 4th century?
Rome is the powerhouse in europe since is never had to deal with the collapse of the empire and the loss of population, knowledge, collapse of cities,…
Rome is a faction with massive standing armies

Also how centralized is the Germanic kingdom? You say like France, but France of what era?

Perhaps France was a bad comparison. It would probably be more like Anglo-Saxon England during the 10th century or the German kingdom of Henry the Fowler or the Ottonians.

 

[Arkenfolm]

Germany in Roman times had around 4 million people on an area far bigger than todays Germany. An agricultural state can't put 30 % of its population under arms like barbarians can, so unless they multiply and cut down their defensive trees and bushes to make room for more fiels you'll have a far weaker politiy than the OTL patchwork of barbarian clans.

Sure it can, because the proposition here is that Germany in Antiquity cannot increase its population too much more than OTL because of the environment relative to technology, but can increase its social complexity to the point of forming a decentralised kingdom.

The only reason the HRE was able to do that was because they were the biggest kid on the block. Against a Roman Empire which spans from the First Cataract to the great North Sea, they'd be dwarfed. Germania would never be a peer power to Rome, because unlike Persia, they don't have the mass. In a total war scenario, it would be a repeat of the Punic Wars; no matter how well the Germanics fight, they just don't have the manpower (or the money), and they'd inevitably be crushed by Rome's superiority in both of those areas.

I don't see how it is much different than the Parthian Empire, which was fairly decentralised and occupied mostly poor and inhospitable terrain and still was one of the greatest enemies of the Roman Empire. The Romans probably could have conquered them if it was worth it, but it wasn't since it was distant and outside of Mesopotamia, not profitable. Ancient Germania is the same--it's distant, a campaign expensive, and it's not profitable. A few more towns serving as administrative centers, a few more pastures for cows and horses, and a few more mines here and there relative to OTL wouldn't sway the Roman opinion. It would be better to nip at the edges when possible and come to an agreement instead of mutually destructive warfare.

I might add that Germania doesn't have as secure borders as you might think: the Rhine is not really difficult to cross with Roman engineering, and aside from the Black Forest and the swampy, (Malaria-ridden!) reaches of the middle Rhine, western Germany is well suited to campaigning with large armies. The Alps are the only major obstacle, really.

Most of the Rhineland was hilly forest like the Eifel, Black Forest, Ardennes, etc. And any crossing of a large river by default channels an army along predictable paths.

I suspect any impression of western Germany as "well suited to campaigning with large armies" owes more to OTL where it was a battleground with expansionistic states like Austria, Burgundy, and France. And it's especially irrelevant before the Middle Ages when it was mostly forest with some Roman forts here and there (which I guess TTL would be Germanic border forts and probably a very profitable area economically for a united Germanic state).

 

[Evil Crusader]

Against a Roman Empire which spans from the First Cataract to the great North Sea, they'd be dwarfed. Germania would never be a peer power to Rome, because unlike Persia, they don't have the mass. In a total war scenario, it would be a repeat of the Punic Wars; no matter how well the Germanics fight, they just don't have the manpower (or the money), and they'd inevitably be crushed by Rome's superiority in both of those areas.

I believe you're pushing it too far. Such a scenario will never see this Germania face the united might of Rome, much like Persia quite never did, and in fact arguably even less likely because this Rome is more, not less, challenged than OTL. Once we sort of butterfly this organisation into existing (which is a large stretch, but I think necessary to the conversation at hand), Rome would find itself quite unable to entirely dismantle the Kingdom, only damage it enough that peace is bought for a decade or three. However...

Sure it can, because the proposition here is that Germany in Antiquity cannot increase its population too much more than OTL because of the environment relative to technology, but can increase its social complexity to the point of forming a decentralised kingdom.

I don't see how it is much different than the Parthian Empire, which was fairly decentralised and occupied mostly poor and inhospitable terrain and still was one of the greatest enemies of the Roman Empire. The Romans probably could have conquered them if it was worth it, but it wasn't since it was distant and outside of Mesopotamia, not profitable. Ancient Germania is the same--it's distant, a campaign expensive, and it's not profitable. A few more towns serving as administrative centers, a few more pastures for cows and horses, and a few more mines here and there relative to OTL wouldn't sway the Roman opinion. It would be better to nip at the edges when possible and come to an agreement instead of mutually destructive warfare.

Most of the Rhineland was hilly forest like the Eifel, Black Forest, Ardennes, etc. And any crossing of a large river by default channels an army along predictable paths.

I suspect any impression of western Germany as "well suited to campaigning with large armies" owes more to OTL where it was a battleground with expansionistic states like Austria, Burgundy, and France. And it's especially irrelevant before the Middle Ages when it was mostly forest with some Roman forts here and there (which I guess TTL would be Germanic border forts and probably a very profitable area economically for a united Germanic state).

If the Kingdom is disorganised, then Rome can take it head on fairly easily, and its only saving grace is whether Persia or internal rebellion are more dangerous. You're also exaggerating a bit how poor and weak were the Parthians, they certainly could not threaten Rome at its zenith, but frankly, nobody else could and at least they survived the apex of Roman power.

 

[Hearkener]

Sure it can, because the proposition here is that Germany in Antiquity cannot increase its population too much more than OTL because of the environment relative to technology, but can increase its social complexity to the point of forming a decentralised kingdom.

An increase in social complexity decreases the number of troops that can be effectively deployed relative to population size. Social complexity just means a higher degree of socio-economic specialization, which is great for overall productivity, but also means that most of the population cannot be levied for war without causing significant economic damage. A decentralized political organization would only worsen this problem, as it means that the gain in productivity will be offset by a lack of effective ressource management at the macro-economic level.

I don't see how it is much different than the Parthian Empire, which was fairly decentralised and occupied mostly poor and inhospitable terrain and still was one of the greatest enemies of the Roman Empire. The Romans probably could have conquered them if it was worth it, but it wasn't since it was distant and outside of Mesopotamia, not profitable.

This is an inaccurate view of ancient Iran. While Babylonia was the economic heartland of the empire, the highlands had major cities and dense agriculture of their own. What you assert is true of the Median plain and the Gedrosian desert, but not of Elam, Bactria, Khorasan and the Caspian coast. (I also disagree that the Romans could have conquered them, but that discussion is better reserved for its own thread some other time.)

Ancient Germania is the same--it's distant, a campaign expensive, and it's not profitable. A few more towns serving as administrative centers, a few more pastures for cows and horses, and a few more mines here and there relative to OTL wouldn't sway the Roman opinion. It would be better to nip at the edges when possible and come to an agreement instead of mutually destructive warfare.

Ancient Germania is not nearly as far away from the Roman heartland as Iran. That aside, Germania could either be a major power or maintain a peace with Rome, these two options are mutually exclusive. Becoming a significant state actor would certainly trigger war with the empire, and Germania will loose this war badly. Whether Germania is profitable is a secondary concern to that of state security - and as Germanicus showed, you didn't need to conquer the region to destroy its military capacity.

Most of the Rhineland was hilly forest like the Eifel, Black Forest, Ardennes, etc. And any crossing of a large river by default channels an army along predictable paths.

This raises a major problem of believing Tacitus and Caesar's word. Afaik, the Eifel was already a highly domesticated landscape during the Iron Age, and so was modern Hesse. Being channelled along predictable path was the default of premodern warfare - there are generally only a handful of approaches viable for a group of several thousand armed men with their supplies. For the Romans, this was not a problem, because the Germans tended to be annihilated in open battle.

 

[Hearkener]

I believe you're pushing it too far. Such a scenario will never see this Germania face the united might of Rome, much like Persia quite never did, and in fact arguably even less likely because this Rome is more, not less, challenged than OTL. Once we sort of butterfly this organisation into existing (which is a large stretch, but I think necessary to the conversation at hand), Rome would find itself quite unable to entirely dismantle the Kingdom, only damage it enough that peace is bought for a decade or three. However...

Even a relatively minor concentration of force on part of the Romans would probably be able to destroy this kingdom. Persia is not a good analogue here, because it was also a massive empire swimming in cash and manpower. Germania has neither of those going for it; I doubt its conquest would be more challenging than that of Macedon or Ptolemaic Egypt.

 

[Arkenfolm]

If the Kingdom is disorganised, then Rome can take it head on fairly easily, and its only saving grace is whether Persia or internal rebellion are more dangerous. You're also exaggerating a bit how poor and weak were the Parthians, they certainly could not threaten Rome at its zenith, but frankly, nobody else could and at least they survived the apex of Roman power.

I would envision the decentralisation more as a style of governance brought on by necessity rather than a situation one might easily exploit. Yes, there would be defectors to Rome, but they need not be numerous nor powerful if the common opinion portrays Rome as being a worse threat to traditional privileges and liberties than the "King of the Germans". Lots of countries have decentralised structures but come together against a common threat.

An increase in social complexity decreases the number of troops that can be effectively deployed relative to population size. Social complexity just means a higher degree of socio-economic specialization, which is great for overall productivity, but also means that most of the population cannot be levied for war without causing significant economic damage. A decentralized political organization would only worsen this problem, as it means that the gain in productivity will be offset by a lack of effective ressource management at the macro-economic level.

A society can increase in social complexity without much increase in specialisation. That's practically every "tribal unification" scenario ever, and some of these unifications produced confederations and empires that lasted for hundreds of years. Many new roles like "government bureaucrat" overlap with the class who would serve as warriors, because the government wants people of moderate status and in Germanic society that means owning livestock and being able to defend your possessions.

This is an inaccurate view of ancient Iran. While Babylonia was the economic heartland of the empire, the highlands had major cities and dense agriculture of their own. What you assert is true of the Median plain and the Gedrosian desert, but not of Elam, Bactria, Khorasan and the Caspian coast. (I also disagree that the Romans could have conquered them, but that discussion is better reserved for its own thread some other time.)

Sure it is. There were plenty of high mountains, badlands, and other environs where farmers didn't live throughout the country. It's why it was fairly defensible. Interestingly, most conquerers of Iran like the Arabs, Turks, or Mongols happened to be from similar environments and were highly mobile.

Ancient Germania is not nearly as far away from the Roman heartland as Iran. That aside, Germania could either be a major power or maintain a peace with Rome, these two options are mutually exclusive. Becoming a significant state actor would certainly trigger war with the empire, and Germania will loose this war badly. Whether Germania is profitable is a secondary concern to that of state security - and as Germanicus showed, you didn't need to conquer the region to destroy its military capacity.

It took the Romans several centuries to destroy Carthage, a far richer territory that posed a far greater and direct threat to Rome. Rome never destroyed Parthia, despite probably being capable, because even at their height they settled with seizing their western vassals and Mesopotamia (the latter of which was only held for a short period). There's no easy naval route to Germania after all.

 

[Evil Crusader]

I would envision the decentralisation more as a style of governance brought on by necessity rather than a situation one might easily exploit. Yes, there would be defectors to Rome, but they need not be numerous nor powerful if the common opinion portrays Rome as being a worse threat to traditional privileges and liberties than the "King of the Germans". Lots of countries have decentralised structures but come together against a common threat.

A society can increase in social complexity without much increase in specialisation. That's practically every "tribal unification" scenario ever, and some of these unifications produced confederations and empires that lasted for hundreds of years. Many new roles like "government bureaucrat" overlap with the class who would serve as warriors, because the government wants people of moderate status and in Germanic society that means owning livestock and being able to defend your possessions.

Sure it is. There were plenty of high mountains, badlands, and other environs where farmers didn't live throughout the country. It's why it was fairly defensible. Interestingly, most conquerers of Iran like the Arabs, Turks, or Mongols happened to be from similar environments and were highly mobile.

It took the Romans several centuries to destroy Carthage, a far richer territory that posed a far greater and direct threat to Rome. Rome never destroyed Parthia, despite probably being capable, because even at their height they settled with seizing their western vassals and Mesopotamia (the latter of which was only held for a short period). There's no easy naval route to Germania after all.

I think you're highlighting secondary or consequential factors as primary and defining when they're not.
While I assume a complex Kingdom for the sake of discussion where, it seems to me, Hearkener reasonably assumes one would not quite be able to emerge, we almost literally have no example of decentralised but complex societies being able to resist Rome without extreme geographical factors, and those would not be in play for such a Kingdom.
Most 'tribal unifications' survived in places 'nontribals' had comparatively little interest in, such as the Iranian plateau, but that did not protect the lowlands of Mesopotamia, the actual ability of the Iranian polities to project power beyond the plateau efficiently enough to challenge Rome did.
Germania doesn't really offer defensible borders, and if they're a complex society, they have plenty of places that could be seized by Rome and the necessity to protect from eastern nomads themselves. The likelihood Rome eventually moves in is very high, they almost did so OTL when there was far less incentive after all.