Septimius Severus attempts to conquer Germania

[Towelie]

Septimius Severus was one of the most militarily active Roman emperors in the late period of the Pax Romana and spent a lot of money in improving the Roman military.

He went on a campaign in Scotland in 208 that seemed not to accomplish much, however. What if he had taken the troops and resources spent on that expedition and attempted to conquer Magna Germania instead?

There are a few reasons that I think this is possible. First, the Roman military in the late 100s-early 200s AD, before the start of the Crisis of the 3rd Century, were at arguably their most powerful point. Severus was a militarily minded emperor with success in battle against a wide diversity of foes. Much like Germanicus 180 years before, I believe that Severus would have no issues defeating Germanic enemies deep beyond Roman borders.

Second, over 200 years of being on a common border with Rome had allowed Germania to become the kind of province that Rome could take and bring into the empire. Unlike at the time of Augustus, there were economic and bureaucratic organizations that Roman law could attach itself to and made administering the area easier. Augustus was with Varus trying to basically lasso jello; there were no institutions that Roman law and Roman economics could attach to. Germania in 200 AD was much more economically and demographically vibrant than before. This would make conquering the province worth it for the Romans. Essentially, it had become like Gaul at the time of Caesar; an economically growing and politically fragmented sprawl of land that had long had mercantile connection with Rome.

Lastly, the political situation in Rome was stable, and would allow Severus to be able to go on this kind of campaign.

Any thoughts on this? Is there another time where Rome could have taken Germania instead you can think of?

 

[hasdrubal barca]

Great thread, Its an interesting idea. It is certainly within the realm of possibility that Severus could subjugate a portion of Germania. If we look at comparable campaigns of Marcus Aurelius and Maximinus Thrax it shouldn't be hard for Severus to repeat their success. So maybe everything west of the Weser along with an extension of the frontier in southern Germania.

 

[LSCatilina]

First, the Roman military in the late 100s-early 200s AD, before the start of the Crisis of the 3rd Century, were at arguably their most powerful point.

Rome just went out of a long and quite violent civil war, tough. Its armies weren't in best shape, and fiscal ressources had declined since the first part of the century. Arguably, it was still pretty much stronger than what was the case in the IIIrd century, but military troubles and short-comings were still becoming to be a real issue.

You'd argue that in spite of that, an expedition in Germania could have been attempted on a technical level, which wouldn't be wrong. So, let's go to other points.

Severus was a militarily minded emperor with success in battle against a wide diversity of foes

Actually, I'm not sure if military minded really covers his politics. He was certainly less so than Trajan, and probably more cautious of the military (that became an important social and political institution) than a "barrack" emperor. The campaigns he devised and led were arguably more about stabilizing borders (African Limes, Persia, Britain) than actually conquering regions.
His campaign in Caledonia, while not hugely successful, was far from disastrous as he managed to clientelize the Lowlands, putting an hand to the border troubles there for some years, and planned to do it again after their rebellion, dying before realizing this project.

Any campaign in Germania would most probably have the same reasoning : securing borders as much as possible against raids, finding a better geostrategical outcome (as along the Tigris).

And...There wasn't that kind of incitative for what matter Germania : Marcus Aurelius did a good job (for all its issues) crushing Marcomans and Quades, and the Rhine wasn't yet the hot border it became later.
Without strategical motivation, or loot motivation (Germanic chiefdoms weren't wealthy enough to cover the costs of such an expedition, let alone gather some for Rome), I don't see why Severus would have pulled something like this.

Much like Germanicus 180 years before, I believe that Severus would have no issues defeating Germanic enemies deep beyond Roman borders.

There's important differences at work there : first, Germanicus could rely on a largely untroubled Romania when it was clearly more divided and prone to military coup in the IInd century.
Then, most fo Germanicus' expeditions were shows of force made to intimidate local chiefdoms into clientelization. But most of Danubian chiefdoms were pretty much clientelized or at least under Roman scrutiny : going even deeper (without much strategical sense) would have been harder. (And, to be honest, would have Germanicus been authorized to go all Caesar on Germania, he would likely have known a somewhat bitter experience).

A more interesting, would it be only because of its chronological closeness, would have been Marcus Aurelius' experiences on Marcommanic Wars : the central Danubian region was more or less clientelized since decades (then known and structured), but after the mid-IInd century, these chiefdoms had to face some pressures from inner Germanic groups (that reinforced or formed new tribal entities) or Sarmatians in the East.

It's not yet the mess that will be in the IIIrd century, but Marcus Aurelius experience points how focused and free from political and fiscal issues, a campaign in Germania should be to hope being successful.
(Arguably, Tiberius had similar problems in the Ist century, when he had to abandon a project of campaigning against Marcomanni because of a revolt in Illyria).

The impoverishment of danubian provinces (that will likely be the departure point of any Roman expedition) was still pretty much perceptible in the late IInd/early IIIrd century, and Septimus would have probably an harder time than Marcus due to less immediate ressources at hand.

Second, over 200 years of being on a common border with Rome had allowed Germania to become the kind of province that Rome could take and bring into the empire.

It really wasn't. While I'm the first to agree that Germanic chiefdoms (simple and complex) along the Roman borders were structurated trough Roman influence and gave the area some stability, Germania was still mostly piss-poor in comparison of other Roman provinces and seen mostly as a peripherical buffer-region that had to be checked in order to prevent Germans to raid the borders.

Contrary to, say independent Gaul, where a very long history of trade and political influence coming from the hellenic time, made the region not only wealthy but having economical and politicals structures Romans could basically fit in, Danubian and Rhineish Germania was certainly less develloped on this regard (to say nothing of inner Germania, that was going to some serious troubles at this point).

Lastly, the political situation in Rome was stable, and would allow Severus to be able to go on this kind of campaign.

Again, I'm forced to disagree : the political situation in Rome was held firmly because Severus never lost an eye on the Senate and other generals (as Plautianus).
Undergoing border operations was a thing (and probably a reason why he leave Caledonia that quickly was that he didn't want to leave Rome to itself too long), but a full-fledged campaign far from Rome and asking for taking winter quarters along the Danube or Rhine is something else.

Now, for all my post (and I think @SlyDessertFox will prove much more detailed informations) I think that Septimus Severus campaigning in Germania is plausible, as long you provide some strategical incitative.
Let's say that the troubles in Central Europe provokes enough changes for that Marcomans do press on the limes as they did later during Gallian's reigns. Rather than providing them a foedus, Septimus Severus could indeed be more inclinded to deal with a (relatively) short campaign to re-establish Roman hegemony over Danubian peoples.

Any thoughts on this? Is there another time where Rome could have taken Germania instead you can think of?

The whole of Germania was simply too big, too underdevelloped, too far from its cores for Rome to swallow it up.

That said, I could see a conquest or heavy clientelization of portion of Rhineish and Danubian Germania along complex lines on Main and Weiser rivers up to Bohemia as it was attempted along Ad Moesia and Dacia under Trajan and the early reign of Hadrian.
How long would it last, is anybody guess, but I could see lasting it up at least up to the end of IInd century.

 

[Avrorrange]

Rome just went out of a long and quite violent civil war, tough. Its armies weren't in best shape, and fiscal ressources had declined since the first part of the century. Arguably, it was still pretty much stronger than what was the case in the IIIrd century, but military troubles and short-comings were still becoming to be a real issue.

Shouldn't the armies be in it's best shape simply because it's just out of the civil war and still having to manpower and finance to sustain it?Armies just out of civil wars are usually highly experienced for the most part and countries generally need to spend the no longer needed veterans elsewhere to distract them

 

[LSCatilina]

Shouldn't the armies be in it's best shape simply because it's just out of the civil war and still having to manpower and finance to sustain it?

Generally, civil wars mean less fiscal resources (having been used if not depleted by rivals) and less military forces avaible (having killed each other). You have to add that Severus did spend quite a lot increasing the wages of the army, and demobilising a singificant part of these armies by providing them lands.
Now, would the greater experience of provincial troops in the region be useful? Indeed, but better experience doesn't mean at the latest better military situation overall.

I think your mistake there is to consider that an army that Septimus would have raised in 208 would have been the exact same than in 190's : Septimus replaced part of his Danubian armies with fresh troops that he payed more. Its armies on this region went trough a big civil war, and had to be largely demobilized (with generally was the case of veterans first) and replaced by other troops while the recruitment of Danubian troops to be located elsewhere in the Empire.
Not that "not in the best shape" mean at the latest it was crumbling, but the military situation under Septimus wasn't what it was some decades before in term of military stability.

I agree, tough, that the expression may be too strong : let's say "his armies weren't in a better shape than Antonine's, rather they were under more stress"

 

[SlyDessertFox]

All of you are right to an extent. It's true that to some extent the Roman Empire was much weaker following the plague and subsequent neglect by Commodus. While the civil wars probably did have a negative effect, Severus did well to bring up the army to an incredibly strong state. Severus increased the Roman army to about 502,000, which nearly 50,000 more troops than Marcus Aurelius had at his disposal at the start of his reign (this is according to Wikipedia's numbers). This caused financial strain, though that wouldn't really start to be felt until after Severus, so it shouldn't be much of a factor here.

LSCatilina is correct in that Severus didn't seem to have much interest in conquest, but rather in securing borders. However, there is a very easy way for him to be able to justify it in this respect. It's not hard for Septimius Severus to reach the same conclusion as Marcus Aurelius and realize the best way to protect Dacia is to expand into Marcomannia. From there it's not that big of a stretch to come to the conclusion that it might be best to connect a border running from the Carpathians straight across to the Elbe as the most defensible.

 

[LSCatilina]

I'm not sure that a full-fledged conquest of the Danubian region north of the Danube and south of Carpathians would have been technically doable or even desirable.

It's a lot of undevelloped terrain, where everthing (roads, forts, supply lines, peacekeeping operations, etc.) was to be done and represent a financial drain for not that good reason. For what mattered Romans, clientelized Danubian peoples served as a protection between Danube and Carpathian mountains.

Apart from the usual opportunistic raids, clientelization of Danubian peoples seems to have worked off relatively well during Septimus' reign. Quadi, Marcomanni, Iazyges didn't really moved against Rome. If Iazyges' exemple is any indication, Septimus probably recruited a lot of his auxiliaries among these peoples in first place.
The region was fairly stable, especially in comparison of the changes happening beyond the Carpathians.

You'd need a particularily cluless Septimus to campaign, without provocation (altough such provocation could be a good PoD IOTL, but not for a full-fledged conquest), in order to expand borders : he'll loose a significant pool of auxiliaries recruitment, destabilize the region (the aftermath of Dacian conquest will look mild and peaceful in comparison) at the point he may prepare the ground for a new Marcomanni War-equivalent (with Vandals and Sarmatians in the first role) for his successor, while risking civil order by being fairly absent from Italy and core provinces of the empires.

Marcus Aurelius attempt at provincialisation is essentially recorded by the Historia Augusta, which should be used with some caution being both a late record by centuries, and having a strong satirical stance. Now, it's possible that Marcus Aurelius did tought about these plans, but if it was the case, it wasn't something clear enough to be worth of mention by Cassius Dio that is one of the most detailed sources on the war.
Rather, the goal of Marcus Aurelius seems to have not to destroy these peoples, but to render them to their previous clientelization (you'd notice that he made many prisoners laeti in the provinces, and in Italy before it backfired in Ravenna).

I agree that Septimus, if the border was deemed troubled and under pressure, would campaign against danubians. But as Marcus Aurelius before him, I think it would be about beating them into submission, as he did in Caledonia IOTL.
Arguably, tough, Septimus could push off peoples as Iazyges a bit, and conquer the part between Danube and Ister, joining up the border between Pannonia and Dacia north of Danube.

 

[SlyDessertFox]

I won't disagree, since it requires a fundamental shift in the situation and in Severus's own personality I think, to bring that scenario about. I was just pointing out the only way it could realistically be justified in this era. A conquest of Germania would be more likely under someone who feels they need a great big military conquest to solidify their rule, or their legitimacy with the army. This would be a much easier scenario to bring about if we were talking about, say, Maximinus Thrax, and not Septimius Severus.

 

[Towelie]

I think I should clarify. I wasn't referring to Severus campaigning against the Marcomanni or any of the groups along Rome's Danubian German frontier. I was more referring to an attempt to expand Roman rule from the Rhine to the Weser, and fortifying the region from the bottom of the Weser to the fortifications in Germania Superior. The reasons for this are that it would bring the Romans into contact with the Lombards and Saxons, on the far side of the river, and perhaps with some luck forestall their later rise, all the while enriching the Empire with the spoils of a successful conquest of the Frisii, Chauci, and what would later become the Alemanni. Agri Decumates would be lost in the chaos of the 3rd Century Crisis, but with such an expedition, this might be avoided by pushing the frontier up and bringing the tribes that would be an issue later on under Roman direct rule.

There would really be no benefit in trying to establish Roman rule in the Danubian regions of Germania, as it would upend imperial diplomacy with the tribes there and create more indefensible borders without real reasoning or benefits behind it.

The areas that Augustus tried and failed to bring into the Empire are essentially what I had in mind. This would put the Romans in control of the North Sea coast up to the Weser, which would offer North Sea trading opportunities (as well as more coast for the Navy to defend against raids, unfortunately).

 

[SlyDessertFox]

I think I should clarify. I wasn't referring to Severus campaigning against the Marcomanni or any of the groups along Rome's Danubian German frontier. I was more referring to an attempt to expand Roman rule from the Rhine to the Weser, and fortifying the region from the bottom of the Weser to the fortifications in Germania Superior. The reasons for this are that it would bring the Romans into contact with the Lombards and Saxons, on the far side of the river, and perhaps with some luck forestall their later rise, all the while enriching the Empire with the spoils of a successful conquest of the Frisii, Chauci, and what would later become the Alemanni. Agri Decumates would be lost in the chaos of the 3rd Century Crisis, but with such an expedition, this might be avoided by pushing the frontier up and bringing the tribes that would be an issue later on under Roman direct rule.

There would really be no benefit in trying to establish Roman rule in the Danubian regions of Germania, as it would upend imperial diplomacy with the tribes there and create more indefensible borders without real reasoning or benefits behind it.

The areas that Augustus tried and failed to bring into the Empire are essentially what I had in mind. This would put the Romans in control of the North Sea coast up to the Weser, which would offer North Sea trading opportunities (as well as more coast for the Navy to defend against raids, unfortunately).

I know, and I was trying to find a rationale for Severus to do it. The only rationale would seem to be a general desire to see strategic expansion in creating a more defensible border.

Otherwise, none of this is needed with Maximinus Thrax, who had both a pretext and political reasons to find invading Germania an attractive option. You can't really get Severus to do that though.

 

[LSCatilina]

I think I should clarify. I wasn't referring to Severus campaigning against the Marcomanni or any of the groups along Rome's Danubian German frontier.

Danubian Germania is where a campaign of Septimus would make the most sense, mostly because it was a region that prooved to be more troublesome than Rhineish Germania at the point, mostly due to being under direct pressure of inner movements.
A campaign in the direction of the Weser or Main in the early IIIrd century makes, strategically, little sense : the region is calm, and would remain so without a PoD in Central Europe.

At best, you could have an earlier troubled Upper Danube, over Raetian borders, as it happened in the early reign of Caracalla (altough the reason for campaigning against Allemani remains unclear), but Rhine simply stopped to be a main strategical concern for Rome since decades n the 200's/210's.
An expedition there would be, as said SlyDessertFox mostly for the sake of expension, or as I'd like to say, for the lulz of it. While less of a concern for Rome, the region directly neighbouring the imperial provinces were a good poll of auxiliaries and laeti, as point the archeological finds about Allemani.

I was more referring to an attempt to expand Roman rule from the Rhine to the Weser, and fortifying the region from the bottom of the Weser to the fortifications in Germania Superior.

You'd have more chances on that with a PoD before Antonines, as with their rule, the military focus clearly shifted on Danube and Persia. Either with a longer rule of Domitian, or a smoother succession, but another campaign against Chatti seems to me the latest plausible PoD avaible for a, modest, expansion of Rome on the Weser/Main basins.

The reasons for this are that it would bring the Romans into contact with the Lombards and Saxons, on the far side of the river, and perhaps with some luck forestall their later rise, all the while enriching the Empire with the spoils of a successful conquest of the Frisii, Chauci, and what would later become the Alemanni.

Lombards would be a really secondary people by the Ist century, being dominated by Marcomanni (and as such already in contact with Rome).
Saxons, however, didn't yet appeared as a distinct confederation : peoples that formed it probably already existed tough.
Alemanni, on contrary, seem to have already emerged as a confederation in the lower Rhine basin : moreover, they were largely present within Germania Superior in the early IIIrd century, essentially romanized people.

I don't see why you think Rome would be interested on conquering Frisii and Chaucii, eventually. As said above, the usual strategical objective of Romans was to beat peripherical into submission, crush their numbers by deporting prisoners well into provinces, enroll the others as auxiliaries (as Usipians in Caledonia) and enact some clear proofs of suzerainty.

I think you're underestimating the level of integration of Germanic chiefdoms and confederacies : most of these didn't grew out of themselves, but as you pointed in your OP, did exist because Rome, well, was a thing and not only influenced them culturally, but had a sorta working relationship with bordering polities. The modus vivendi between Rome and Germans (that, again, were largely present in the western provinces as laeti and auxiliaries) wasn't just coincidental, but was institutionalized by Romans themselves (which then helped structurte Germanic complex chiefdoms) : to throw out this institutionalized relationship would ask for more than just "we want to get direct contact with people we have already have contact with".

On this regard, a deeper expedition against Chatti in the late Ist century would be a good motivation and a plausible PoD, if Domitian survives, to have extended Agri Decumati on Weser/Main basins.

Agri Decumates would be lost in the chaos of the 3rd Century Crisis, but with such an expedition, this might be avoided by pushing the frontier up and bringing the tribes that would be an issue later on under Roman direct rule.

Again, most of neighbouring peoples were either clientelized or integrated within Germania Superior or Inferior as laeti, auxiliaries or even some equivalent of foederati. For all it's worth, Alemanni were partially under Roman direct rule, in the Agri Decumati : their confederation is more or less tied to this political fact.

It's what make me doubt that extended Agri Decumati could hold the IIIrd century crisis : because Barbarians were an integral part of it, demographically and politically.
At worst, you'd end up having them forming a different confederation or league than Franks with Upper Rheinish and/or Danubian peoples instead of Lower Rheinish peoples.
At best, you'd end up having Alemanni on steroids which could make Agri Decumati turning more like a foedus, rather than just giving up on it, and maybe make Alemanni playing the role of Franks ITTL, as relatively trusted federates.

There would really be no benefit in trying to establish Roman rule in the Danubian regions of Germania, as it would upend imperial diplomacy with the tribes there and create more indefensible borders without real reasoning or benefits behind it.

I entierly agree : but that's as well the case of Rheinish borders. If anything, the relation between Romans and Germans there was even stabler due to less pressure from migrating groups in the late IInd century.
You'd even have more incitative, with a PoD during Septimus' reign, to have an expedition in Danubian Germania because most of the troubles happened there and that it wouldn't be far-fetched to have some of them happening in the 200's/210's. On contrast of Rhineland Germania that didn't knew such kind of pressure at this point and not before some decades.

The areas that Augustus tried and failed to bring into the Empire are essentially what I had in mind.

It's not really clear what Augustus attempted to take, and I rather think he had no clear idea either. While Romans certainly set up a direct rule as far as Dortmund, what existed beyond a line going (roughly) from Munster to Wruzburg looked a lot the relation ship that existed between Rome and Sequani earlier, as in a sort of alliance/clientelization, than a provincialisation : the lack of clear geographical or political limits certainly helped the confusion.

Not that a Roman control of part of Germania beyond the Rhine is unthinkable : but it would be relatively hard comparatively to the few advantages, economical or strategical (altough, as SlyDessertFox pointed, it could serve as a political victory).

Augustus had something that Domitian didn't have tough : in the Ist BCE, Germania was a big unknown for Romans, and disrupting the local political balances had less impact that it had one century later. Rome at some point needed as much a stable relation with Rhineish polities than these depended from it.

In a really short attempt to illustrate the possibilities...(sorry for Frisia, BTW, I'm still finishing the overall map)

1) Situation in the IInd century
2) What Domitian could realistically expect from an extended Agri Decumati
3) What Romans effectively controlled, roughly, in the Ist century BCE.

This would put the Romans in control of the North Sea coast up to the Weser, which would offer North Sea trading opportunities (as well as more coast for the Navy to defend against raids, unfortunately).

Rome was already pretty much in contact with North Sea trade ensemble, it's more or less why they went for Britain IOTL.
As for maritime raiding, the point is not they needed more coast (especially coasts than like in Frisia, were particularly swampy and unfit the development of coastal fleet, not without massive development that even Britain didn't have IOTL), it's that IIIrd century Romania couldn't afford it being stuck in a loop of coups, raids, raids and coups.

 

[Towelie]

I don't think that it can be argued that conquering Rhenish Germania would have been an undertaking with a lot of solid reasoning for it. But the Romans up until Caesar didn't imagine campaigning and occupying in Belgica either, and the reasons for such an action in this POD would be that it is an easier conquest than trying to do something like link Roman Dacia with the bend of the Danube near Aquincum or enslave the Marcomanni, it would have enough wealth and prestige behind it to serve as a political boon for Severus, and that it would please the army, which really was the chief priority of the Severan dynasty if we are being honest.

What I am more interested in however is whether or not it is possible. I gave a lot of thought to the best point in time that Rome could have had to have gotten a frontier on the Weser rather than the Rhine, and I figured that Roman military power necessary for such an act did not signifcantly wane until the Crisis of the Third Century, but that Germanic polities were not well developed enough to make Roman governance a feasible endeavor until you started seeing the rise of the Confederations in the third century, but that it had to be done before these confederations were powerful enough to resist a concerted Roman effort. I figured that Septimius Severus had the best chance, but it could very well have been Alexander Severus who had an even better one, or possibly if Marcus Aurelius had designated a better successor than Commodus, than they could have done it. But the window for Roman conquest I think is not a large one. Trajan might have been able to conquer the region relatively easily, but his conquests did not seem to stay conquered for long, and I doubt it would have been any different in the case of Germania.

 

[LSCatilina]

But the Romans up until Caesar didn't imagine campaigning and occupying in Belgica either

With the slight difference that Gaul was known by Romans not only trough merchants and political influence, but as well detailed accounts such as Poseidonios'.
Eventually, Gaul in the Ist century BCE was more develloped, more structured and more "compatible" Roman presence than Germania, at the possible exceptions of immediate Rhineland and the region between Weser and Danube which were still pretty much inhospitable for all what Romans could think.

Belgica itself was a relative tough nut to crack, especially due to its relative backwardness compared to the rest of Gaul : it still looked like a walk in the park in comparison of Augustean campaigs in Germania, and still probably would do so in the Ist century AD with the massive exception that, this time, Romans would be aware of how difficult for how few advantages.

enslave the Marcomanni,

So far, never said that : for my part, I stressed that, at the contrary, a Roman campaign on Danubian Germania would be to maintain Marcomanni and their hegemony in Roman clientele trough forced satellization and deportation of laeti. Which, eventually, would be the only plausible outcome of a Rhineish campaign as well.

and that it would please the army, which really was the chief priority of the Severan dynasty if we are being honest.

Germanic chiefdoms were relatively poor (so no big expectation for loot) and their lands were fairly uninteresting for veterans settlement purposes (Decumates Fields, as pointed above, were essentially inhabited by Romanized Barbarians).
How Septimus maintained the army happy was by increasing their wages and demobilising veterans as soon as he could, which are relatively independent of campaigning (if something, long campaigns in Barbaricum could hinder that).

What I am more interested in however is whether or not it is possible.

By Septimus' reign, it's technically possible, but geopolitically implausible : there's simply not much incitative do launch a limited conquest campaign just because Rome can do, at the risk of really disturbing the local balance of power and making, from a relatively peaceful border, a troubled one.
The only incitative that anyone could think of for a Septimus' campaign along the Rhine would be for the lulz, sorry for prestige, and you have simply a lot of other, better regions to do so would it be because not only it wouldn't impair Septimus' prestige (warring for no good reason generally made Roman elite frown upon, would it be at the sight of wasted money) but actually be useful.

Caledonia was IOTL (both the historical expedition, and the one that was planned before Septimus' death), you could pretty easily find one in Danubian Germania or Ad Moesiam, and of course Persia. If there was ONE border of Romania that didn't need intervention at this point, it was the Rhine.

As I don't think your objective is realistically doable in the early IIIrd century, and following your OP requirement, I'd tried to point you to a PoD that seems to me more workable in the mid-late Ist century AD : the souvenir of Augustus' defeat in Germania is fresh enough, you had strategical motivation to eat up a bit of Rhineish Germania, and a possible political motivation to do so.

I gave a lot of thought to the best point in time that Rome could have had to have gotten a frontier on the Weser rather than the Rhine

It's possible, but would be a quite complex border to maintain : it's not a big deal when the Rhine border is fairly calm as for the IInd century and an extended Decumates Fields northwards and eastwards is certainly maintenable for some decades at least. That is not really a point of contention, but rather a point of discussion : what changes this extended DF would bring?

that Germanic polities were not well developed enough to make Roman governance a feasible endeavor until you started seeing the rise of the Confederations in the third century

The rise of Germanic confederations largely answered the problem of outer pressures and middle-scaled migrations in the Barbaricum. It's not really about a more important sophistication (even if I gladly concede that it lead to a greater sophistication overall, due to increased acceptance of economical and social mobilisation) than what existed in the Rhine. Long story short, Germanic chiefdoms (simple and complex) even when client aren't a stepstone to roman diploannexation à la Europa Universalis. It's an already integrated part of Romania (roughly a set of buffer zones) that, while being significantly poorer and underdevelloped, play a strategical role.

Eventually, Romans simply couldn't be bothered to swallow up these regions except (and that's where a PoD of your convenance can take place) when these chiefdoms simply can't (or don't want) to fulfill this role by openingly revolting for exemple. In the IInd century, Rhineland chiefdoms didn't, being somewhat content with the current modus vivendi. On the other hand, you could use the rebeliousness of the Chatti confederation in the Ist century to have Romans making a point and directly controlling their lands.

I figured that Septimius Severus had the best chance, but it could very well have been Alexander Severus who had an even better one, or possibly if Marcus Aurelius had designated a better successor than Commodus, than they could have done it.

More you go in the IIIrd century, more Danubian borders are seen as the soft underbelly of the Empire. There's a reason why Severi generally favoured Danubian armies, and why barrack emperors were often from Dalmatian or Illyrian origin.
Of course, Alexander had to fight the Allemanic confederation, but he died because he wanted to warp it up quickly, as it was obviously not the main military concern. (Even Maximinus did acknowledged that).
It doesn't help that the army becomes more and more of a social and political institution with the time.

So, I really think that in order to fulfill your objectives, you need to look out on the Ist century. It would be not only more possible and easier, but objectively plausible.

Trajan might have been able to conquer the region relatively easily, but his conquests did not seem to stay conquered for long, and I doubt it would have been any different in the case of Germania.

It's why I think that extended Decumates Fields could easily turn to a western Ad Moesiam-equivalent : a mix between clientelized and military regions without clear geographical relief to rely on when it comes to defense against raids, and soon abandoned.
It's not bound to happen this way, tough, as proposed with an extended Decumates Fields that could turn in a de facto foedus managed by an Allemanic equivalent made of IOTL proto-Allemanic and Chatti. I admit this is a best case option.

 

[Karolus Rex]

Septimius Severus was one of the most militarily active Roman emperors in the late period of the Pax Romana and spent a lot of money in improving the Roman military.

He went on a campaign in Scotland in 208 that seemed not to accomplish much, however. What if he had taken the troops and resources spent on that expedition and attempted to conquer Magna Germania instead?

There are a few reasons that I think this is possible. First, the Roman military in the late 100s-early 200s AD, before the start of the Crisis of the 3rd Century, were at arguably their most powerful point. Severus was a militarily minded emperor with success in battle against a wide diversity of foes. Much like Germanicus 180 years before, I believe that Severus would have no issues defeating Germanic enemies deep beyond Roman borders.

Second, over 200 years of being on a common border with Rome had allowed Germania to become the kind of province that Rome could take and bring into the empire. Unlike at the time of Augustus, there were economic and bureaucratic organizations that Roman law could attach itself to and made administering the area easier. Augustus was with Varus trying to basically lasso jello; there were no institutions that Roman law and Roman economics could attach to. Germania in 200 AD was much more economically and demographically vibrant than before. This would make conquering the province worth it for the Romans. Essentially, it had become like Gaul at the time of Caesar; an economically growing and politically fragmented sprawl of land that had long had mercantile connection with Rome.

Lastly, the political situation in Rome was stable, and would allow Severus to be able to go on this kind of campaign.

Any thoughts on this? Is there another time where Rome could have taken Germania instead you can think of?

Severus was, like you, said one of the most militaristic Emperors of the pre-Crisis of the Third Century period, but that was more thanks to circumstances.

His increase of spending in the army and the expansion of the army numbers, was more because he needed the men to keep the Empire under his rule, than to be used in expanding the Empire. He also raised the pay of the legions. Now all of this look like good ideas but bigger army numbers and a bigger pay begun to have an effect on the economy and on the Imperial treasury.

To expand into Germania would imply having to raise new legions to garrison the area, and not only to protect the new border but also to ensure that the new province was calm.

The region also didn't had much to offer that the Roman would consider economical beneficent to them. Of course one can point out that conquest can be only because a Emperor wanted to gain prestige, and thanks to that they ended up with the Britannia, the black hole of resources.

The expansion into Germania also creates another problem.

To fight the tribes on the border, and to ensure that they were never strong enough to pose a threat to the Empire, the Roman often gave gifts of gold to the tribes behind the border ones. The influx of gold allowed chieftains to gather more retinues and more weapons. This tactic was used to incentive war between the germanic tribes. OTL the problem began when the tribes they had been feeding reached the frontier and displaced, or subjugated, the former border tribes.

Also Severus in 208 was 62. A Germanic conquest would take many years and if he dies at the same time as OTL then it isn't enough time to establish a viable presence, and we don't know if his sons would want to keep investing men and gold in Germania.

I think I should clarify. I wasn't referring to Severus campaigning against the Marcomanni or any of the groups along Rome's Danubian German frontier. I was more referring to an attempt to expand Roman rule from the Rhine to the Weser, and fortifying the region from the bottom of the Weser to the fortifications in Germania Superior. The reasons for this are that it would bring the Romans into contact with the Lombards and Saxons, on the far side of the river, and perhaps with some luck forestall their later rise,

The Romans were always in contact with the tribes of the Weser river. They actively supported them to encourage war in Germania so that the border tribes would be forced into a war in two fronts.

Also expanding into the Weser would be counterproductive as now they would had to find new tribes behind the ones that were now in the border, so that they can continue their policy of keeping the Germans divided and in constant war between themselves.

all the while enriching the Empire with the spoils of a successful conquest of the Frisii, Chauci, and what would later become the Alemanni. Agri Decumates would be lost in the chaos of the 3rd Century Crisis, but with such an expedition, this might be avoided by pushing the frontier up and bringing the tribes that would be an issue later on under Roman direct rule.

The Empire would hardy enrich with spoils from Germania. The only thing they could bring in large quantities would be slaves.

There would really be no benefit in trying to establish Roman rule in the Danubian regions of Germania, as it would upend imperial diplomacy with the tribes there and create more indefensible borders without real reasoning or benefits behind it.

The areas that Augustus tried and failed to bring into the Empire are essentially what I had in mind. This would put the Romans in control of the North Sea coast up to the Weser, which would offer North Sea trading opportunities (as well as more coast for the Navy to defend against raids, unfortunately).

The Empire's interest in the North Sea was very limited and the only thing I can image that the romans would be interested in controlling would be the Amber Road, but given that they were the ones with the greatest economical power, the trade routes all went to Rome so no real reason to waste resources in Germania when other borders needed attention.

I don't think that it can be argued that conquering Rhenish Germania would have been an undertaking with a lot of solid reasoning for it. But the Romans up until Caesar didn't imagine campaigning and occupying in Belgica either, and the reasons for such an action in this POD would be that it is an easier conquest than trying to do something like link Roman Dacia with the bend of the Danube near Aquincum or enslave the Marcomanni, it would have enough wealth and prestige behind it to serve as a political boon for Severus, and that it would please the army, which really was the chief priority of the Severan dynasty if we are being honest.

If Severus had wanted real expansion and conquest he would do it in areas were it would be economically viable in the short term, if he had wanted that the could had conquered Mesopotamia and restore the province. This would give economical benefits on the short term in contrast expansion into the Rhine would force a massive investment to make the new province economically viable.

Belgica was conquered during the Gallic Wars. The political and economical situation are completely different.

What I am more interested in however is whether or not it is possible. I gave a lot of thought to the best point in time that Rome could have had to have gotten a frontier on the Weser rather than the Rhine, and I figured that Roman military power necessary for such an act did not signifcantly wane until the Crisis of the Third Century, but that Germanic polities were not well developed enough to make Roman governance a feasible endeavor until you started seeing the rise of the Confederations in the third century, but that it had to be done before these confederations were powerful enough to resist a concerted Roman effort. I figured that Septimius Severus had the best chance, but it could very well have been Alexander Severus who had an even better one, or possibly if Marcus Aurelius had designated a better successor than Commodus, than they could have done it. But the window for Roman conquest I think is not a large one. Trajan might have been able to conquer the region relatively easily, but his conquests did not seem to stay conquered for long, and I doubt it would have been any different in the case of Germania.

Possible yes, but it would be hard.

It would need an Empire at peace in most borders, a large uncommitted army, good leaders and large amounts of manpower, resources and gold. If Britannia needed 4 Legions and an equal number of auxiliaries, Germania would need, at the very least, the same number and a very competent general that is also a very competent administrator and loyal beyond doubt.

Severus could had done it, he had most of the above, but he was old. If he died in the middle of the campaign, probably Caracala would keep the war but the moment problems begin in the Persian border the campaign will go to an halt, a quick peace deal is made and the Emperor and the legions are off to Asia, because no Emperor would allow a General to conquer something in fear he would turn against him.