The year is 1024 Ab urbe condita (271 CE). The Roman Empire is in crisis.
In the east, Zenobia, Queen-Regent of Palmyra, has led a highly successful revolt against the Empire and taken over the provinces of Aegyptus and Syria-Palaestina, cutting off the empires lucrative eastern trade and its Egyptian breadbasket. The fall of the great city of Antiochia the previous year has left the gates of Anatolia open, and Zenobias forces pour into the mostly undefended territory.
In the west, Postumus, governor of Germania Inferior, has declared himself Emperor of a so-called Gallic Empire and successfully captured all of the Empires European territory north and west of Italy, including the fertile regions of Gallia Narbonensis and Hispania. The defeat of a rival at Moguntiacum (Mainz) in 1021 has left the Gallic Empire strong and united,[1] although Postumus has shown no signs of a desire to capture Rome itself.[2]
[1] After the fall of Moguntiacum OTL, Postumus refused to allow his soldiers to plunder the city, and they murdered him, leading to the rapid collapse of the Gallic Empire. In this timeline, he has survived to 1024.
[2] OTL and in this timeline, the governor of Mediolanum (Milan) declared for Postumus in 1020, but Postumus did not march into Italy to support him, preferring to solidify his control over his territories. As the motivations for Postumus are hazy OTL, this timeline assumes a desire to achieve a military dictatorship without ties to a corrupt Rome.
Ruling in Rome is the Emperor Quintillus, the first Emperor successfully imposed by the Senate against the wishes of the army in some time. In a battle at Aquileia late in 1023, Quintillus defeated the usurper Aurelian (or, rather, Aurelian was killed by a stray arrow from his own forces, taken as an omen to support Quintillus) and is currently involved in reestablishing order in Italy. However, Senatorial factions control many of the actions of the Emperor to their own benefit. The Emperors attentions are currently occupied by a revolt of mint workers in the city of Rome itself, which some allege has the backing of many Senators.[3]
[3]Quintillus OTL was defeated by Aurelian at Aquileia in 1023 after ruling for only a few months. Aurelian, being a great military general, would go on to reunite the Empire, but Quintillus, being a solid administrator but middling general, will not successfully defeat Postumus or Zenobia. The revolt of the mint workers was real and captured Aurelians attention for most of 1024.
Fast forward three hundred and fifty-one years.
The year is 1375 Ab urbe condita[5], although the city of Rome no longer glimmers like it once did. In the years 1010-1050, the Empire entered a state of complete collapse from which no ruler could save it; eventually, separate Emperors came to rule much-reduced portions of the Empire, each without the manpower to invade, overcome and defeat one another.
[5]622 CE. Note (for reference) that this would mark the earliest years of Islam in OTL. Although butterflies have eliminated Mohammed, the potential for religious conversion and conquest is strong across the Arabian peninsula.
By 1125, pressure from migrating Hunnic nomads forced the Germanic tribes across the borders of the northern empires, and Germanic armies quickly overran the Italian peninsula and much of Southeastern Europe; Rome was sacked in 1134. The Gallic Empire, strongest of the successor empires, managed to hold the line along the Rhine River until the coming of the Huns, but the rest of the former Empire in Europe was broken. Small Germanic kingdoms sprang up, often claiming some Roman heritage.
This brief stability lasted barely more than a half-century, as the Huns trailed closed behind the Germans and around 1190 launched their first attacks against the Empire. In 1201, a disastrous battle at Augusta Trevorum (Trier) wiped out most of the army of the Gallic Empire, and the Huns poured into the last Roman state of Europe, quickly destroying much of the Roman presence in Gallia. The Huns preceded to attack the German states of Southeastern Europe from their new base, although Roman-ruled authorities in Hispania survived. By 1220, the Hunnic Empire has collapsed to the winds, but the memory of its devastation would stick with the remnants of Europe.
The fall of Rome to Gothic[6] forces had caused the retreat of the legitimist Roman Emperors to Africa, where they established a somewhat ironic capital at Carthago. This empire in Africa would suffer occasional invasions by the Italian Goths, and later by the Vandals, who overran post-Hunnic Italy. Still, it survived, and by 1375 has managed to regain control of parts of Sicily from the quarreling Vandal lords.
[6]Unlike OTL, there was no obvious Visigothic/Ostrogothic split. Rather, various military leaders in a number of different groups led the Goths; later historians estimate between seven and twelve separate kingdoms. The group that sacked Rome consisted of three kings banded together, one of whom would declare himself King of Rome after killing the other two. Other Gothic kings established themselves at Aquincum, Singidunum (Belgrade), Aquileia and Augusta Vindelicorum (Augsburg), all of which fell to the Huns, as well as various smaller kingdoms in southeastern Europe and possibly other states about which records are lost.
The Palmyrene Empire under Zenobia reached its height shortly after her death under her son, the Emperor Vaballathus. The empire had captured the key city of Byzantium and threatened an invasion of Graecia, but the plans for invasion were cut short by a Sassanid invasion. Zenobia had defeated the Sassanids decisively in 1032, but twenty years had passed and they had regrouped. Zaballthus successfully turned back the assault, but the Palmyrenes were forced to retreat from Anatolia, which entered something of a power vacuum. Although both sides repeatedly attempted invasions of one another, the Palmyrenes and Sassanids remained at rough equality from 1060 onwards, and Palmyra stands more or less unchanged from its earlier years in 1375, although the collapse of the Romans has left the Palmyrenes much poorer due to a lack of trading partners.
Graecia and Anatolia existed in something of a power vacuum after the precipitous decline of Rome and the retreat of Palmyra. To some extent, the Greek city-states reemerged, although these nominally subjected themselves to the Emperor at Rome (or, occasionally, Palmyra). The withdrawal of the Roman army from the area made it easy prey for the Goths and the Huns, and civilization has been reduced to ashes north of Attica in European Greece. The Greeks of Ionia have fared better, and small states dot the landscape, with Ephesus dominant and the most powerful. Inland Anatolia also contains small states with complex alliances to keep them safe against Palmyrene and Sassanid incursion. (The Palmyrenes and Sassanids are too concerned with one another to seriously attempt an invasion of Anatolia anyway.)
Hispania was the only part of the Gallic Empire to survive the barbarian invasions. Following the defeat at Augusta Trevorum, the Emperor retreated to Lugdunum (Lyons) and then eventually to Tarraco (Tarragona), which became the capital of the much-reduced empire. In theory, the Franks who have since occupied Gallia owe allegiance to the Emperor at Tarraco, but in fact they are independent. Still, Baetica and Tarraconensis remain well-developed and fertile regions, as well preserved as Africa or Aegyptus. Lusitania is less well developed and has been settled in parts by a Germanic people called the Suevi, but Hispania still has its Roman heritage and a possible future.
The Franks and Burgundians have occupied Gallia in the power vacuum left by the Huns; the Franks dominate in the north, while the Burgundians control the southeast. Both states are weak and highly decentralized, and the collapse of the Gallic Empire has left almost no Roman infrastructure behind. All cities of the territory have been reduced to rubble or tiny towns, and the region is unlikely to become consequential for at least a century.
As the years wore on, the Gallic Empire slowly abandoned Britannia; by the time of the Battle of Augusta Trevorum, only a token garrison at Londinium kept it under Gallic control. The various Celtic peoples first overran Britannia, but they were soon overrun themselves by Anglo-Saxons, who now control the lowlands. Britannia, never having possed the infrastructure of Gallia in the first place, is in an even worse state, and future historians know almost nothing about the region in this period.
Religion-wise, most of the former Empire remains pagan. The collapse of Roman authority in the east cut off many sources of Christianity. Christians do have some numbers in the Palmyrene Empire, where they comprise about 20% of the population. However, the success of the Palmyrene Empire in keeping the region stable has prevented Christianity from going viral among the lower classes, and in fact the religion is now in decline from its peak about a hundred years prior as many of its millenarian predictions have failed to come true (and lack of state sponsorship has prevented further expansion). The one area where Christianity has influence is in the Greek states, some of which have completely converted. Generally speaking, Ionia is more receptive than Greece proper; Athens has banned the practice of Christianity, although this is unique and owes to Athenss traditional reactionism. Ephesus, the most powerful Greek state, remains under pagan administration, but the inner Anatolian states are mostly Christian, and even influential Miletus has come under Christian rule.
Outside of the Mediterranean Basin and Europe, the world is mostly unchanged, although, as mentioned earlier, Mohammed has been butterflied away. (The strong support for the Palmyrenes among the Levantine cities would make a momentous rise of the Arabs much less feasible in this timeline anyway.)