Thanks for the feedback. I've thought about this a bit and did some research and here's how I think things might develop were this to happen.
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484- Peroz, the Persian Shahanshah, leads an army of the empire's most elite soldiers against the enroaching threat of the Ephthalites in the east. The Ephthalite chief Akhshunwar, realizing his forces can't stand against the heavily armored lancers of the Persians, has a huge trench dug and maneuvers Peroz to give battle at the field. When Peroz leads his army in a charge against the Ephthalite forces, they plunge into the trench and the Ephthalites fall upon them, slaughtering almost all of them and plundering the bodies and supply train. Akhshunwar, seeing the elite of the Persian forces dead and the Sassanid Empire open, leads his forces in a full invasion of Persia.
485-488- The Ephthalites steadily advance westward, capturing strongpoints along the way and killing all the provincial governors they can catch, each of them blood relations of the Sassanid Dynasty. The campaign culminates with the Ephthalite siege and capture of Ctesiphon in 488 that brings about the final downfall of the Sassanid Dynasty. The royal treasury is looted and Akhshunwar takes up temporary residence in the palace.
489-503- Ephthalite forces ride across the former Persian empire and hunt down the remaining members of the Sassanid bloodline. Some of the former governors of border provinces manage to flee west to Byzantium where some Ephthalites attempt to follow, but soon retreat from Byzantine territory. With the Sassanids thoroughly thrown out of Persia with some remnants in Byzantium, as the Kidarite remnants fled Bactria for Peshawar in their earlier conquest, the Ephthalites set up a Tegin, or viceroy, in Ctesiphon, who directly rules Mesopotamia and Armenia, as they set one in Gandhara (present day NW Pakistan) earlier but rule Persia themselves and include the territory in their migrations back and forth across their empire, taking tribute from the various minor nobles. The Ephthalites are largely acceptable to the majority of their subjects due to their worship of Zoroastrianism, despite the fact that they bury or entomb their dead instead of leaving them exposed to the elements, and, despite a few local Ephthalite rulers, let the religious minorities, like the Buddhists of Bactria and the Christians in Persia, worship freely and without persecution. A permanent Ephthalite capital, Piandjikent, is founded during this time in the Zaravshan valley 65 kilometers southwest of Samarkand. It's hardly ever used as the 'true' capital is a mobile nomad camp.
491-502- Anastasius I rises to the Byzantine throne. He immediately exiles Longinus, the brother of his predecessor Zeno I, to Egypt and expels other Isaurians from Constantinople. This provokes an Isaurian revolt and a rebel army led by Longinus of Cardala advances into Phyrgia. Shortly after the Isaurian army is soundly defeated at Cotyaeum in 491, some Isaurians seek refuge with the new arrivals from the east, the Ephthalites, while others wage guerrilla warfare from their Isaurian strongholds. The Ephthalite Tegin in Ctesiphon, who knows that his people played such political maneuvers with the Persians before the Sassanids' fall, sends assistance to the Isaurians. The Ephthalite nomads easily penetrate the frontier and make their way to Isauria while more specialized Mesopotamian forces manage to take some border fortresses and cities, such as Theodosiopolis. Ephthalite forces manage to lift the sieges of various Isaurian strongholds by surprised Byzantine forces in 493. In 494, a new Isaurian army, with Ephthalite assistance, advances once again into Phyrgia where another battle is fought. It is a narrow victory by the Byzantines, but the battle depletes both forces enough that both sides are forced to retreat and recuperate. Both armies continue to march across Anatolia for the next four years until Anastasius I arranges to pay tribute to the Ephthalites and agrees to cede some border territory in 498. The Ephthalites withdraw from Byzantine territory and the Isaurians are once again reduced to guerrilla warfare from their strongholds in Isauria. The revolt is finally crushed in 502.
503-517- In India, the Gupta Empire has been declining ever since a war of succession followed the death of Skandagupta and tributary kings and tribes have been reasserting their independence. The final fall of the Gupta Empire arrives with an invasion by the Ephthalite forces of the Tegin of Gandhara in 505. The last remnants of the Kushans and Kidarites are destroyed in the process and, after an Ephthalite victory over a Gupta army at Eran in 511, northwestern India comes under the rule of the Tegin of Gandhara, now the ruler of his own autonomous empire within the greater Ephthalite Empire.
In Byzantium, Anastasius I precariously holds on to power as religious riots frequently break out amongst the orthodox extremists who are intolerant of the other sects and the fact that Anastasius himself is a convinced Monophysite. He attempts to hold to a moderate policy, but the repeated demonstrations drive him to abandon it and adopt a Monophysitic program. His subsequent unpopularity in the European territory of the empire leads to a rebellion lead by an ambitious man named Vitalian who is assisted by a horde of 'Huns'. The Ephthalites take the oppurtunity to extract yet more tribute from Anastasius to keep them quiet in the east while he deals with the rebellion in the Balkans. The rebellion is crushed in 515.
Despite the tribute he is forced to pay to the Ephthalites, Anastasius maintains a good administration and does his best to build up the treasury while reforming taxes to bring relief to the people. Through his careful fiscal policy, he still manages to leave a respectable amount of gold in the treasury.
In the main nomadic portion of the Ephthalite Empire, outside of the Mesopotamian and Gandharan Teginates, the frontier of empire is pushed further east into the Tien Shan range and Ephthalite embassies are sent to Wei, a kingdom in northern China.
518-527- Anastasius I dies in Constantinople. His general and commander of the palace guard, through having the only troops in the city and gifts of money, is able to secure his accession to the Byzantine throne as Justin I, establishing the fall of the Theodosian Dynasty and the rise of the Justinian Dynasty. A career soldier, Justin has little knowledge of statecraft and surrounds himself with trusted advisors, the most prominent of which is his nephew Flavius Petrus Sabbatius whom he adopts as his son and invests with the name Justinian. In 525, Justin repeals a law that effectively prohibited a member of the senatorial class from marrying a woman from a lower class of society, which helps to make Justinian's marriage to a former mime actress named Theodora more acceptable and eventually results in a major blurring of the old class distinctions at the Imperial court. The latter years of his reign are marked with strife with the Ostrogoths which forces Justin to continue to pay tribute to the Ephthalites, making Byzantium effectively a tributary to the Ephthalite Empire. The embarassment of the situation, as well as the tenuous finances of the treasury due to the tribute payments to the Ephthalites, leaves its mark on Justinian during his adoptive father's reign. In 526, Justin's health is visibly beginning to fail and he names Justinian his heir in April of 527. A few months later, Justin dies and Justinian rises to the throne as Justinian I, later to be known as Justinian the Great.
528-532- In India, the Ephthalite Gandharan Tegin's bloody policies, especially against Buddhists and their monasteries, provokes a coalition of Indian princes against Ephthalite rule. Ephthalite forces are defeated and they begin to rapidly lose ground in the subcontinent.
Upon Justinian I's accession to the throne, he has one goal in mind: revenge against the Ephthalites and to match the conquests of the Roman Emperor Trajan in the 2nd Century. The Empire has always had troubles in the Balkans during the reigns of his predecessors and Justinian is adamant that nothing will distract from a campaign in the East, not even the Byzantine dream of the reconquest of the Western Roman Empire. Before the campaign is launched, Justinian issues a first draft of the Corpus Juris Civilis in 529 which codifies Roman law, to be followed by supplementary collections of new laws.
The Mesopotamian Tegin continues to demand tribute and, when Justinian refuses, begins sending Ephthalite nomads raiding across the Byzantine-Ephthalite border. A Byzantine general named Belisarius successfully combats the Ephthalite raids, raising him in the estimation of Justinian. Belisarius' place is secured when he puts down the Nika rebellion of 532 in Constantinople, securing Justinian and Theodora's places on the thrones. Justinian places Belisarius in charge of the force that will reconquer the territory taken by the Emperor Trajan and given up by his successor Hadrian.
533-545- Belisarius and his army invade Ephthalite Armenia and quickly take Theodosiopolis, then inflicts a major defeat on the Mesopotamian Tegin's forces outside of Artaxata, putting all of Armenia in Roman hands. Belisarius then advances into Mesopotamia where he inflicts another major defeat on the Ephthalites at Mosul. He then conquers the Tegin's, and the former Sassanids', capital Ctesiphon in 536 and then the port of Ublra on the Shatt al Arab (not sure what it was called then) in 540. At this point, with the Slavs starting to make more and more frequent raids into the Balkans, Justinian offers the Ephthalites a setttlement: cede the current conquests to the Empire as well as half of all their treasure. Since Armenia and Mesopotamia would be brought directly into the Empire, Belisarius agrees with the proposal wholeheartedly as he passes it on towards the Ephthalite chief. The Ephthalite 'capital' proves to be in Persia proper at the time and the chief receives the defeated Mesopotamian Tegin and Justinian's settlement within a short period of time. The Tegin is sent to serve under the Gandharan Tegin and the Ephthalite chief refuses Justinian's settlement as he sends his army riding into Mesopotamia. Belisarius wages an inconclusive campaign against the Ephthalites across Mesopotamia but finally manages to inflict enough damage to their horde to bring the chief to renegotiate Justinian's settlement in 544. After numerous consultations with Justinian, a settlement is finally reached in 545 in which the Ephthalites will pay a quarter of their treasure and cede Armenia and Mesopotamia to Byzantium. Belisarius returns to Constantinople in triumph, having pushed the Empire's eastern border to where the Emperor Trajan had in the 2nd Century (Roughly equivalent to present day Iran's borders with Iraq, Turkey, Armenia and Azerbaijan).
546-559- Justinian, fearful of Belisarius' power upon his triumphal return to Constantinople, sends him to the Danube to combat the raids of the Slavs. He also starves Belisarius of supplies and reinforcements so that he is forced to the defensive and isn't able to exploit the victories he wins against them. As Belisarius is stuck in the Balkans, the Ephthalites decide to take advantage of the situation and once again attack into Mesopotamia in 548. Justinian, not trusting Belisarius enough to give him the resources necessary to combat the Ephthalites, decides instead to send old eunuch Narses, who helped Belisarius in suppressing the Nika rebellion and whom Justinian trusts. Narses proves to also be a gifted general and soundly defeats the Ephthalites at Ctesiphon and drives them back into Persia proper a ways before another settlement is reached in 552. The Ephthalites pay another quarter of their treasure, effectively matching Justinian's former deal, and agree to an 'Endless Peace'. In return, Narses returns to Mesopotamia and the Empire takes no further territory. Still, Narses wants more than to just be someone who helped retain Belisarius' conquests and convinces Justinian to let him campaign in Arabia. With Mesopotamia in Byzantine hands, they won't have to worry about an eastern neighbor maneuvering against them amongst the bedouin of Arabia and the region can be brought safely into the Empire. In 555, Narses launches an amphibious invasion from the Egyptian Red Sea ports of Myos Hormus and Berenice and captures the ports of Yanbu and Jidda before moving inland and conquering the important trade cities of Medina and Mecca, finishing the conquest of the Hejaz in 559. During the course of the campaign, Narses exchanges embassies with the Christian naval power of Axum who controls Yemen in southern Arabia. Relations between Byzantium and Axum remain friendly, even though the Byzantines now hold a greater sphere of control in the Red Sea and now have Persian Gulf ports which will mean greater competition in the Indian trade.
In the Balkans, Belisarius faces the first raid by the Bulgars across the Danube into Byzantine territory in 559 and defeats them, driving them back across the river.
560-565- Narses remains in the Hejaz as prefect while Belisarius stands trial in Constantinople in 562 regarding a trumped up charge of corruption. He is judged by his bitter enemy, his former secretary Procopius of Caesarea, and is found guilty and imprisoned. Not long after, Justinian pardons him, orders his release and restores him to favor at the Imperial Court. Justinian and Belisarius dies within weeks of each other in 565, the two men who helped restore the former limits of the eastern Empire while Narses expanded it beyond. Justinian's nephew rises to the throne as Justin II.
In the Ephthalite Empire, a new chief rises to replace his predecessor that presided over the loss of northwestern India, Armenia and Mesopotamia. This new chief, who has spent much time in Persia proper, recognizes the threat to the empire with the Indians in the south, the Byzantines to the west and the Turkic tribes migrating to the east and north. He decides that copying the peoples they have conquered to some extent, and giving them a stake in the empire, is the best thing to do. He moves his permanent capital to Piandjikent and brings in Persian and Bactrian architects and artists to increase the splendor of the city, which has seen much profit from lying along the Silk Road. He also abolishes the Tegin of Gandhara and bring his territory under his direct rule. Measures are passed that further centralize the Ephthalite government and Ephthalites, as well as some priveleged Persians and Bactrians, are granted noble titles and the dihqans, the petty landholding nobility, are brought back into the government system as the backbone of provincial administration and tax collection instead of the previous 'tributes'. The mobadan mobad, the head of the Zoroastrian church, is brought to Piandjikent even as the Buddhists and Christians are allowed to worship in the city as well. Within the early years of his reign, the advance of the Indians is halted and the border held at roughly the Indus River and Gandhara.
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Thoughts?