A possible argument against this proposal is that a development and centralisation of state structures in an independent Britannia may eventually lead to the formation of a regional power that can cause trouble for Rome. I have some doubts as to that. First of all, if Rome never conquers the island, that means substantially less (direct) contact with Rome, and thus less of an impetus to centralise and develop state structures. In OTL, it was typically the contact with Rome (either by fighting the Empire or by being vassalised by it) that sparked such processes. Furthermore, the Romans would simply use the same diplomatic strategy they used in Germania in OTL: play local rulers off each other, distribute bribes, maintain trading relations (which would become as integral to the British economy as to the Romans), and intervene militarily when necessary. This is a far better (and cheaper) situation than having to permanently station multiple legions there— and it maintains most of OTL's economic benefits via trade. Such a strategy is hardly fool-proof, so an occasional punitive expedition to enforce Roman interests and ensure the compliance of local rulers should be expected. Well, dealing with some troublesome Celtic rulers across the sea from time to time is still far cheaper than peanently stationing Britannia with more troops than could reasonably be justified.
I will respond at some point in the near future to push back on the idea of Britain being a net loss, but I will note that every Roman border territory saw state development inspired by Rome. paying off local rules and distributing bribes didn't stop the Goths from being a problem.
In fact, let me propose an alternate scenario. During the *Third Century Crisis, a Roman-influenced, Christian king from England invades Gaul and the Rhineland. His name...
Artorīgos. The Romans will call him Artorius, and the rest is (Alternate) History.
pqyite[A Roman force, even a mere auxiliary force, is going to be considerably more expansive— and thus more threatening to any would-be raider. The Vikings were a threat because their victims were relatively weak. Raiders trying to take on the Roman Empire will find that
they are the weak ones in this particular situation.[/quote]
If the Romans were so capable of dealing with raiders, how come you had the massive fortifications along the Saxon Shore? I don't know if we can say those fortifications worked, but they suggest defeating the raiders required a lot more than just a fleet showing up.
You don't need to be able to predict the future to understand what a strategically sound border looks like. Throughout their history, the Romans repeatedly demonstrated that they understood this. A conquest of Germania up to the Elbe is not something that made zero sense. Indeed, clear-thinking military minds repeatedly argued in favour of it.
Do you have a cite for this? August seemed to expand everywhere he could; south of Egypt, into Arabia Felix, etc.
[qquote]The main advantage here is that having those areas under control gives Rome a strong strategic buffer on the northern periphery. Germania beyond the Elbe now consists of petty vassalised tribes who can't realistically grow into a threat.[/quote]
Why can't they become a threat, just like the alemanni became a threat? What evidence is there that the Roman empire would think about this in the context of "fall back lines" along major rivers?
Mesopotamia was incredibly wealthy. In fact, if retained, it would easily be the wealthiest region of the entire Roman Empire... by far. It was the wealthiest region of the Parthian Empire as well. If properly integrated, it will at once become the single greatest benefit to the Roman treasury. Yes, conquering and permanently defending the region is going to be expensive. But there is no way that any even vaguely realistic cost you can care to set as the "price tag" for all this can come even close to matching the huge profits. Economically speaking (obviously not politically), you could basically give up everything west of the Adriatic, including Rome itself, in exchange for Mesopotamia... and you'd benefit from that deal. (Again, economically speaking. The socio-political ramifications are vastly different.)
How does Rome keep this territory? Trajan's conquests were not permanent, and he was not the only Roman to invade and sack Mesopotamian cities. And when the Romans trashed the Parthians enough to bring them down, the replacements, the Sassanids, were an even bigger threat!
Given these factors, could the Parthians (or any of their ATL successors) retake Mesopotamia? It has been argued that the manpower base of Persian armies didn't rely on Mesopotamia. The Persian cavalry armies came from the steppe and the highlands. Even without Mesopotamian wealth, might a poorer but martially powerful steppe empire then succeed in driving back the Romans? I have my doubts about that idea. The simple fact is that "it's the economy, stupid!" (As always.) The cavalry armies may not be recruited from Mesopotamia, but historically, they were fed and paid for by Mesopotamia.
A small band of nomadic archers seizing a fertile floodplain? It is to laugh, is it not?
Which Empire is going to leave a general with such a large army in the east?
Basically, Rome had a trade balance problem. Gold flowed to Persia, to India, to China... and never came back. Whatever we may say about a trade balance issue in the modern age of fiduciary currency, in the days that gold was money, it presented a real danger. There are many theories as to the "decline and fall" of the Roman Empire. I personally think that the outflow of gold was a major cause. The gold supply dwindled, currency got debased, prices inflated as a result, an inflationary cycle got started, etc. Eventually, this led to dire straits, economically speaking. Taxes got raised to such a level that many common people could not bear the burden (the problem being that taxes had to be paid to the government in actual gold, while the Emperors issued increasingly debased coin).
Which economic historians now believe that the specie outflow destroyed the empire? Serious question.