Adrianople Avoided

As you pointed out, the west will be much stronger with no Frigidus and also no Magnus Maximus. Even if the Rhine does freeze over, there's no reason to assume any invasion will get anywhere. The invading germans were already heavily bloodied by the Franks OTL, so here the Romans can just conduct mopping up operations. Constantine only came into Gaul BECAUSE of the incursion, sensing an opportunity to play off of Stilicho's plight. Otherwise, the British usurpation, if it even happens, will just peter out like most British attempted usurpations did.

I'm not sure about both Theodosius' becoming magister ultriusque militum-that is way too much concentration of power into one family for the imperial family to feel comfortable.
 
Well, marriage ties might count for a lot. Or, the Theodosii might pull strings to get it done. Or, we just wait until the Elder dies.
 
Well, marriage ties might count for a lot. Or, the Theodosii might pull strings to get it done. Or, we just wait until the Elder dies.

It can certainly be done once Theodosius the Elder dies, but having a son and father essentially in control of the armies of the east and west is too dangerous for any emperor to contemplate. Even if they won't do anything while said emperors are in power, on their deaths its basically inviting them to take power for themselves.
 
It can certainly be done once Theodosius the Elder dies, but having a son and father essentially in control of the armies of the east and west is too dangerous for any emperor to contemplate. Even if they won't do anything while said emperors are in power, on their deaths its basically inviting them to take power for themselves.

And if said Emperors are mere puppets?
 
And if said Emperors are mere puppets?

Gratian wouldn't just be a puppet of the Theodosians. And since he would be overseeing Valentinian II's being set up as co-emperor, and could reasonably expect to be in practice the one controlling both halves of the empire, you can't really say Valentinian II could be a puppet of the Theodosians either. Gratian may well appoint one of the Theodosians as magister militum for Valentinian II, but having them both as magister militum's of east and west would be too risky for Gratian to do.

Edit: And who's to say Theodosius The Elder still won't be a victim of the post-Valentinian power jockeying?
 
Coming back to this to flesh things out a bit:

AD 370s
A large group of Goths migrate into the Roman Empire. The situation is delicate, but they are successfully pacified and given asylum in several locations across the empire.

AD 380s
Valentinian, Augustus of the West, dies in AD 380. His 21 year old son, Gratian, is proclaimed Caesar of the east, under his brother, Valens, while his younger son, Valentinian II, is proclaimed Augustus of the West at age 9, as a puppet under the Frankish general Merobaudes and a junta of other military officers. Valens, Augustus of the East, dies in 387, and succeeded by Gratian. Merobaudes has been supplanted by Stilicho, the new Magister Militum of the West. In the East, Theodosius the Younger is Magister Militum.

AD 400s
After constant, but relatively minor, raiding by the various Germanic tribes, a large scale invasion is initiated over the course of the winter, as a loose confederation of tribes under the leadership of a Vandal king, Gunderic. After a few brief skirmishes in which the loyalties of Stilicho, also of Vandal extraction, were cast into doubt, he was executed and replaced by Alaric, a Roman commander of Gothic ethnicity. A full year of battles are required to eventually defeat the Vandal confederation, most of the various allied tribes are settled in Roman territory, and the Vandals are forced back across the Rhine to serve as a buffer against the Huns. At the end of 407, Gratian dies, and the Eastern Empire comes to his son by Flavia Maxima Constantia, Constantius III.

AD 410s
Valentinian II dies, and is succeeded by his nephew, and son of Theodosius the Younger, Arcadius. Arcadius, like Valentinian II was in his youth, was a mere puppet of the Magister Militum, Alaric. The Western borders continue to face constant pressure, and most of Alaric's time is spent racing from the Rhine to the Danube and back again.

AD 420s
Alaric dies and, after a brief power struggle among the military, one Flavius Aetius becomes Magister Militum. His term in the office will be dominated by conflicts with the Huns, with whom he has nominally good relations. Constantius III dies and is succeeded by his brother, Constans II as Augustus of the East.

AD 430s
Civil war consumes the Western Empire after the death of Arcadius. Multiple claimants are elevated and the frontiers, already very porous, are threatened greatly as the Huns take advantage of the opportunity. Britannia is overrun by Saxon raiders, and the Franks expand their holdings in northern Gallia during the chaos.

AD 440s
Theodosius, son of Alaric and Galla Placidia, daughter of Theodosius the Younger and Galla (daughter of Valentinian), wins the support of Aetius and is proclaimed as Augustus. His ascension, though he's certainly of the Imperial family, marks the beginning of the so-called Gothic dynasty. Over the course of the decade, the remaining claimants are crushed and Theodosius reigns unopposed. Constans II dies and is succeeded by his nephew, Olybrius.

AD 450s
Theodosius and Aetius defeat a major Hunnic army, after which Aetius is exiled so as to consolidate Theodosius' hold on power. Theodosius then goes on to restore Roman authority in Britannia. By the end of the decade, the Vandals, Alemanni, Burgundians, and Suevi, are all either completely defeated or incorporated into the Empire. The Franks are still semi-independent in the north of Gallia, and a large contingent of the Goths are still independent in the Balkans, between the two halves of the Empire.

AD 460s
The Hunnic Empire collapses after a second defeat, this time by the Eastern Empire. Olybrius is killed in the battle, opening up a power vacuum and leading to intermittent civil war in the East over the coming years. Theodosius campaigns against the Franks in order to solidify Roman authority over them. Agreeable tribes are left in place, while the more intransigent Franks are dispersed through the Empire. Theodosius dies in 467, and is succeeded by his son, Romulus Maximus.

AD 470s
The Sassanid Persians take advantage of the instability of the Eastern Empire and invade. With the frontiers in the West relatively stable for the first time in living memory, Romulus Maximus marches an army to the assistance of his Eastern colleague, Anastasius. The combined armies of East and West defeat the Sassanids outside of Amida, and a status quo peace is secured. In 476, Anatasius dies, and Romulus Maximus succeeds him, ruling over a united Roman Empire.
 
Interesting. I think Romulus would promote a Caesar at least in the east (or west, depending on where he wants to rule).

I'm sure he will appoint a Caesar, but I'm not positive that he'll have significant authority. The way I see it, the standard "The Empire was too big for one man" line is generally a gross oversimplification. In times of crisis, that was definitely true. But when things are relatively calm, its doable (see, for example, 31 BC-AD 235, with honorary mentions going to Lucius Verus and Marcus Aurelius, and Gaeta and Caracalla). Hell, look at the Tetrarchy. It only truly worked when Diocletian was clearly in charge over everyone else. Personally, I think the problem is that the Empire is just barely too large to always be ruled by one man, but small enough that the option is always lingering.

PS, in my fleshed out century there, several individuals are analogues to their same-name counter parts, such as Flavius Aetius and Arcadius. Same name, same time period, same parents, but not the same person.
 
To expand, in very broad strokes:

6th Century
The Roman Empire settles into a century of relative peace and prosperity. The Hunnic threat was no more, and the major barbarian tribes to the north were the Bulgars, Avars, and various Slavic groups, but none were a grand confederation as had existed in the previous century. The Emperors largely took this time to consolidate the Empire, improve the infrastructure, restore important fortifications along the frontiers, and reform the Imperial administration and legal system, among other attempts to keep the Empire strong.

Towards the end of the century, conflict with Persia would flare up to a greater degree than had been customary, and the Sassanids were able to bring great pressure against Roman holdings in the east, while the Avars were able to overwhelm the Danube frontier. However, while the Empire was pressed to maintain its borders, another power in the south was growing.

The Arabian peninsula had never truly been united. The west was generally on good terms with Rome, with the Christian Ghassanids being Rome's major ally, in the north of the peninsula. The eastern coast fell more under Persian influence, with their allies, the Lakhmids generally opposite the Ghassanids. However, to the south, along the coast south of the Ghassanids and into Arabia Felix, there was a growing power uniting the Arabs politically and religiously.

The Ethiopian kingdom of Axum had increasingly become a maritime power, and had repeatedly made efforts to expand its influence into Arabia, and the reach of their Miaphysite missionaries spread beyond their borders. A charismatic preacher out of Yathrib, Ali, began to win over enough converts that he upset the local leaders and he fled to the nearest Axumite garrison. Returning with Axumite troops, Ali began the conquest of Arabia. Soon, all of the people of the Arabian peninsula, save for the already-Miaphysite Ghassanids who would be incorporated diplomatically, were under the nominal control of the king of Axum.

With this new Ethiopian-Arabian kingdom being forged, the dynamic of the Roman-Persian struggles were irrevocably changed. In the contemporaneous war between the two powers, Axum sided with its fellow Christian realm of Rome, enabling the pre-existing border to be maintained. However, tension immediately developed between the two powers.

Previously, Axum had been too remote for Rome to be concerned with doctrinal differences. Now that they had the military might of Arabia under their control, those differences played a key part in the diplomacy between the two powers. The Roman territories now bordering Axum, Egypt, Syria, and Palestine, happened to include many Miaphysite Christians among their populations. On the one hand, the Axumites were on good terms with Rome. On the other hand, they were independent, and now many of the Empire's citizens could be said to have more in common with a foreign power.
 
7th Century
As Ethiopia-Arabia (now ruled from the bustling port of Adulis, rather than Axum) consolidated its hold on the Arabian Peninsula and the Horn of Africa, thus totally controlling the Red Sea trade, a respite from the border wars between Rome and Persia began to settle in. While not as powerful as either of the great Empires to its north, the new realm was powerful enough to influence their policy. If Persia looked too aggressive, they'd make overtures to Rome, and if Rome was too ambitious, they'd treat with Persia. Ethiopia-Arabia did its best to maintain the balance of power during this period, hoping that peace would ensure the vitality of the trade on which it so heavily relied. It proved to be a prudent policy.

Of course, both the Roman and Persian governments were conflicted by this new roadblock in their continual wars, overjoyed that they could reallocate their forces to other fronts, but dismayed that they could not pounce on the first sign of weakness in the other.

For their part, the Persians campaigned against the new steppe menace they faced, the Gokturks. The steppes were hard to control and, other than alleviating the risk the Gokturks presented to the Sassanids, there was little tangible gain to be had. Efforts were made to push into Bactria and India, and, at its best, the Sassanid border managed to reach the Indus river.

Meanwhile, the Roman Empire sought to secure its northern frontier. While the overall frontier was relatively secure, those provinces bordering the Danube in particular were always under constant threat from the Avars. The barbarians posed no real threat to Constantinople itself, but it was considered disgraceful that the environs of the capital were consistently at risk of being raided.

So, a campaign was launched to recover Dacia proper and its coastal regions, in order to provide additional security for Thrace. Upon being reconquered, immense fortifications were built along the Rivers Prut and Dniester, as well as through the Carpathians, as a bulwark against future invasions. The Avars, who had been divided by civil war, were a broken power, the core of their territory seized by Rome. The Sclaveni (Slavic) tribes took over the western remains, while the east became dominated by the Bulgars and Khazars.

The relative ease of the reconquest of Dacia emboldened Rome and new campaigns were planned to push the frontier further. Inspired by Augustus's goal of a frontier along the River Elbe, the Romans pushed into Germania, fighting against the Saxons. Success came not nearly as easily as it had in Dacia, and it took well over a decade before the outcome was assured. Still, by AD 700, Germania west of the River Elbe could be honestly claimed to be pacified, and the European frontier was now roughly 2300 miles long, where it had been nearly 300 miles longer at the beginning of the century.

The government soon realized that it had the manpower and finances to continue pushing the border east, and, if all went well, have a combined European border only half as long as the classical frontier had been. It was almost poetic for the Roman Empire to enter an expansionist phase at this point, mirroring the normally insular Chinese in their efforts to subdue their barbarian neighbors.
 
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