Ah, right - I hadn't fully considered that. But wasn't that how the late empire defended itself? Frontline limitanei with field comitatus I didn't realize that it was the general pattern.
It eventually did, while it depended a lot of
the region concerned.
I always thought Trier was originally a town all of its own? (Treverorum?) though I don't know much about Koln.
Augusta Trevorum was a town, and one of the larger in Gaul eventually. But it was
built around a camp. With
Koln/Cologne (Colonia), Xanten (Castra Vetera/Colonia Ulpa), you have the 3 main cities of the region.
But the importance of these towns were directly tied being military centers, and the region a border.
I'll concede that maybe the fast response legion isn't as useful as I'd hope, but it'd be a "Free Hand" North of the alps.
Oh, that is most certainly doable. It was more or less the case with the Seventh Legion.
Yeah, my thoughts were that the Rhine could be repaired/reoccupied if the Elbe was completely lost - as in, we've given up on Germania for now kind of loss. Not two active frontiers.
Well, it would certainly be an option, but without the historical focus it recieved...It would be clearly less efficient than historically, and it wasn't exactly the Great Wall of China even then.
Most of the work on Rhine border was made after Germania was definitively considered uninteresting, so if the said conquest is made in the Ist century, it would be less about repaired than building most of it.
Now, with the aformentioned expanded Wesser border (yes, I repeat myself, I know), Rhine would be close enough to still be part of an imperial focus and would work far better as a withdrawal position.
Fair enough I suppose. I wouldn't really disagree with that, but it does mean that those who'd invade Britannia may have to deal with Romans closer to home, which could make Britannia safer (a side point at best).
Most invasions prior the Vth century involved Gaelic and Pictish peoples, unfortunatly
. (That's the point where you probably mentally shout "Oh come on!")
Now, obviously, Saxon piracy would be less of a problem, but I'm not certain Scandinavians wouldn't take their place.
See, the common theme with those from my memory is that none of them would
a) Shorten the Border - the Marcomanni Wars, Iazges and Dacia together would have been needed.
But again, shortening geographically the border wouldn't have meant a secure border.
Ad Moesia, for exemple, was constantly under pressure of Roxolani and Free Dacians. Ever tried to build fortifications and roads under unfriendly fire? Me neither, but they did try, didn't managed to and called it a day. It was simply less costly to turn Iazyges and Roxolani into clients dedicated to protect the border.
Marcomannic Wars. Well, again Marcus Aurelius did tried to create provinces of Sarmatica and Marcomania, at least according Historia Augusta that is admittedly to be taken cautiously (while I don't see clear reason to doubt this precise claim).
But the same problem happened : constant war doesn't give near enough stability to not only conquer but structure the territory. Romans fought for 20 years, reaching modern Slovakia, trying to get rid of constant revolts, raids (basically, the Marcomanic Wars were initialy about Sarmatians, but Marcomani used the distraction to raid Romania. Romans eventually tried to lead a conquest war on two fronts. When one was defeated, the other attacked when it was not a roman usurper that rebelled).
Eventually, it was too costly keeping in control northern danubian territories, and shortening geographically the border was simply not worth that effort.
That's the problem : at some point, they couldn't advance without outstretching your forces, and despite what they wanted, they had an hard time keeping it, let alone turning the region suitable for their needs.
b) Had a suitable frontier (like the Rhine). Agri Decumates doesn't seem to follow any natural path, but instead "What works".
They tried to go gradually using when they could
natural paths (Main or hillfoots), because it actually shortened not only the border, but allowed to build roads and infrastructures to react and dispatch more easily troops by
not using the obvious natural path of Alps and mountainous passes, which is more the point than "let's reach this river because it would look more cool".
I'd stress it again like a broken juke-box : what was relevant was less the border than the capacity of roman troops to intervene more quickly.
I mean, Gaul would have been a disaster without the Rhine as a suitable boundary.
I'm far less sure : while it wouldn't have been as convenient than Rhine, the West/East orientation of most great gallic rivers provided usable borders. Most obvious is Loire river, at the point it became a some of a symbolic border.
The absence of great obvious border in Narbonensis (except Cevennes, admittedly), didn't prevented the province to be where Teutoni were eventually defeated, while Italian natural borders didn't do much good on this regard.
I'll give you that's a
border case, but remember that the road network in independent Gaul was already present (and was a factor for Caesarian armies quick progression). A border along a different natural features (and again, they did before and after the roman conquest, formed political borders) could have been reached without being catastrophic thanks to that.
Caesar goal was less geostrategic, than conquering whatever he could (at least half of the Gallic Wars is about strategical opportunism), creating a cohesive geographical unit out of blue with what he took.
Or conquered JUST the nice stuff, but not the crap that was needed to secure it. (I.e. Dacia, or Mesopotamia - though Mesopotamia would always have been difficult).
Dacia is a bad example, giving they really backed the limits on Carpathian foothills and placing
their valli on natural features.
(Mesopotamia is a bit weird, but have to be understood in the context of annexation of Roman and Persian client states, that still heavily based themselves on Tigris and Euphrates, with these being porous anyway)
Perhaps they never had the opportunity, and if that is the case, then that is the case, I am arguing on a highly abstract level here.
They had the opportunities, but it would require more than military successes, it would require both a paradigm change and a PoD that would allow a lesser pressure on peoples living in modernday southern Germany.
1) The Main, then the Weser as you've suggested - each conquest is smaller than the Carpathian Basin, it creates a new set of (shorter) natural borders to maintain - and can rely on the infrastructure on the Rhine to begin supporting it once the roads are established
I was less thinking using Main and Weser as natural borders, than using them as support for structures such as valli, for a more flexible border
2) The Carpathian Basin in its entirety (no small task), but there are plenty of resources for the taking there.
Most of them
being located in the historical province of Dacia, hence why they didn't go too much further and abandoned the eastern part (that and Roxolani pressure).
Keeping it longer would be doable, as much as keeping "moesian" Dacia, especially getting rid of most of Parthian Wars, IMO.
For the rest, giving the two hardly butterfliable events of the Third (such as epidemics and climatic change), with inflation growing up (and even with ressources cashing up, going to be a big problem sooner or later), I'm not sure there is time or possibility before a crisis. Maybe later, but that's a bit besides what the OP ask for.
But yeah, more than 2 cents, but I thought I'd respond
Well, there's mines