Sassanid Empire in the context of "Muslim conquests of Afghanistan"

In the year before the First Council of Constantinople in 381, Nicene Christianity became the official religion of the Roman Empire when Theodosius I, emperor of the East, Gratian, emperor of the West, and Gratian's junior co-ruler Valentinian II issued the Edict of Thessalonica in 380, which recognized the catholic orthodoxy, as defined by the Council of Nicea, as the Roman Empire's state religion. Historians refer to this state-sponsored church using a variety of terms: the catholic church, the orthodox church, the imperial church, the Roman church, or the Byzantine church, with some also used for wider communions extending beyond the Roman Empire. The Eastern Orthodox Church, Oriental Orthodoxy, and the Catholic Church all claim to stand in continuity from the Nicene Christian church to which Theodosius granted recognition. Political differences between the Eastern Roman Empire and the Persian Sassanid Empire led to the separation of the Church of the East in 424. A doctrinal split within the imperial church led to the independence of early Oriental Orthodoxy, while the fall of the Western Roman Empire initiated the gradual separation between Eastern and Western Christianity, culminating in the East-West schism of 1054. The Western Church evolved into the Latin Catholic Church while the Eastern Church that remained under the patronage of the Eastern empire and evolved into the Greek Orthodox Church.

Earlier in the 4th century, following the Diocletianic Persecution of 303–313 and the Donatist controversy that arose in consequence, Constantine the Great had convened councils of bishops to define the orthodoxy of the Christian faith and to expand on earlier Christian councils. A series of ecumenical councils convened by successive Roman emperors met during the 4th and the 5th centuries, but Christianity continued to suffer rifts and schisms surrounding the theological and christological doctrines of Arianism, Nestorianism, Miaphysitism, and Dyophysitism. In the 5th century, the Western Roman Empire decayed as a polity; invaders sacked Rome in 410 and in 455, and Odoacer, an Arian barbarian warlord, forced Romulus Augustus, the last nominal Western Emperor, to abdicate in 476. However, apart from the aforementioned schisms, the church as an institution persisted in communion, if not without tension, between the East and West. In the 6th century, the Byzantine armies of the Byzantine Emperor Justinian I recovered Italy and other regions of the Western Mediterranean shore. The Byzantine Empire soon lost most of these gains, but it held Rome, as part of the Exarchate of Ravenna, until 751, a period known in church history as the Byzantine Papacy. The early Muslim conquests of the 7th–9th centuries would begin a process of converting most of the then-Christian world in the Levant, Middle East, North Africa, regions of Southern Italy and the Iberian Peninsula to Islam, severely restricting the reach both of the Byzantine Empire and of its church. Christian missionary activity directed from the capital of Constantinople did not lead to a lasting expansion of the formal link between the church and the Byzantine emperor, since areas outside the Byzantine Empire's political and military control set up their own distinct churches, as in the case of Bulgaria in 919.

The Byzantine Empire's history is generally periodised from late antiquity until the Fall of Constantinople in 1453 AD. From the 3rd to 6th centuries, the Greek East and Latin West of the Roman Empire gradually diverged, marked by Diocletian's (r. 284–305) formal partition of its administration in 285, the establishment of an eastern capital in Constantinople by Constantine I in 330, and the adoption of Christianity as the state religion under Theodosius I (r. 379–395), with others such as Roman polytheism being proscribed. Although the Western half of the Roman Empire had collapsed in 476, the Eastern half remained stable and emerged as one of the most powerful states in Europe, a title it held for most of its existence. Under the reign of Heraclius (r. 610–641), the Empire's military and administration were restructured and adopted Greek for official use instead of Latin. While there was an unbroken continuity in administration and other features of Roman society, historians have often distinguished the Byzantine epoch from earlier eras in Roman history for reasons including the imperial seat moving from Rome to Constantinople and the predominance of Greek instead of Latin.

The borders of the Empire evolved significantly over its existence, as it went through several cycles of decline and recovery. During the reign of Justinian I (r. 527–565), the Empire reached its greatest extent after reconquering much of the historically Roman western Mediterranean coast, including north Africa, Italy, and Rome itself, which it held for two more centuries. During the reign of Maurice (r. 582–602), the Empire's eastern frontier was expanded and the north stabilised. However, his assassination caused a two-decade-long war with Sassanid Persia which exhausted the Empire's resources and contributed to major territorial losses during the Muslim conquests of the 7th century. In a matter of years the Empire lost its richest provinces, Egypt and Syria, to the Arabs.

Medieval Islamic astronomy comprises the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age (9th–13th centuries), and mostly written in the Arabic language. These developments mostly took place in the Middle East, Central Asia, Al-Andalus, and North Africa, and later in the Far East and India. It closely parallels the genesis of other Islamic sciences in its assimilation of foreign material and the amalgamation of the disparate elements of that material to create a science with Islamic characteristics. These included Greek, Sassanid, and Indian works in particular, which were translated and built upon.

Islamic astronomy played a significant role in the revival of ancient astronomy following the loss of knowledge during the early medieval period, notably with the production of Latin translations of Arabic works during the 12th century.

Marcus Aurelius Carus (c. 222 – July or August 283) was Roman emperor from 282 to 283. During his short reign, Carus fought the Germanic tribes and Sarmatians along the Danube frontier with success.

He died while campaigning against the Sassanid Empire and is believed to have died of unnatural causes. It was reported that he had been struck by lightning. He was succeeded by his sons Carinus and Numerian, creating a dynasty which, though short-lived, provided further stability to the resurgent empire.

Muscat (/ˈmʌskæt, ˈmʌskət/ MUSK-at, -⁠ət, US also /ˈmʌskɑːt/ -⁠aht; Arabic: مَسْقَط, romanized: Masqaṭ, pronounced [ˈmasqatˤ] ) is the capital and most populous city in Oman. It is the seat of the Governorate of Muscat. According to the National Centre for Statistics and Information (NCSI), the population of the Muscat Governorate in 2022 was 1.72 million. The metropolitan area includes six provinces, called wilayat, and spans approximately 6,500 km (2,500 sq mi). Known since the early 1st century CE as a leading port for trade between the west and the east, Muscat was ruled successively by various indigenous tribes, as well as by foreign powers such as the Persians, the Portuguese Empire and the Ottoman Empire. In the 18th century, Muscat was a regional military power: its influence extended as far as East Africa and Zanzibar. As an important port town in the Gulf of Oman, Muscat attracted foreign traders and settlers such as the Persians, the Balochs and the Sindhis. Beginning in 1970, after the accession of Qaboos bin Said as the Sultan of Oman, Muscat experienced rapid infrastructural development; it developed a vibrant economy and became a multi-ethnic society. The Globalization and World Cities Research Network classifies Muscat as a Beta-level Global City.

The Hajar Mountains dominate the view from Muscat. The city lies on the Arabian Sea, along the Gulf of Oman, near the strategically important Straits of Hormuz. Low-lying white buildings are a typical feature of its architecture. The city's port district of Muttrah, with its corniche and harbour, are at the north-eastern edge of the city. Muscat's economy is dominated by trade, petroleum, liquified natural gas and porting.

The Diocese of the East, also called the Diocese of Oriens, (Latin: Dioecesis Orientis; Greek: Διοίκησις Ἑῴα) was a diocese of the later Roman Empire, incorporating the provinces of the western Middle East, between the Mediterranean Sea and Mesopotamia. During late Antiquity, it was one of the major commercial, agricultural, religious and intellectual areas of the empire, and its strategic location facing the Sassanid Empire and the nomadic tribes gave it exceptional military importance.

The Avar–Byzantine wars were a series of conflicts between the Byzantine Empire and the Avar Khaganate. The conflicts were initiated in 568, after the Avars arrived in Pannonia, and claimed all the former land of the Gepids and Lombards as their own. This led to an unsuccessful attempt to seize the city of Sirmium from Byzantium, which had previously retaken it from the Gepids. Most subsequent conflicts came as a result of raids by the Avars, or their subject Slavs, into the Balkan provinces of the Byzantine Empire.

The Avars usually raided the Balkans when the Byzantine Empire was distracted elsewhere, typically in its frequent wars with the Sassanid Empire in the East. As a result, they often raided without resistance for long periods of time, before Byzantine troops could be freed from other fronts to be sent on punitive expeditions. This happened during the 580s and 590s, where Byzantium was initially distracted in the Byzantine–Sasanian War of 572–591, but then followed up by a series of successful campaigns that pushed the Avars back.

Valerian (/vəˈlɪəriən/ və-LEER-ee-ən; Latin: Publius Licinius Valerianus; c. 199 – 260 or 264) was Roman emperor from 253 to spring 260 AD. He rose to power during a particularly unstable period during the Crisis of the Third Century and appointed his son Gallienus as co-emperor.

Valerian is known as the first Roman emperor to have been taken captive in battle, captured by the Persian emperor Shapur I after the Battle of Edessa, causing shock and instability throughout the Roman Empire. The unprecedented event and rumors of his humiliation at the hands of the Persian emperor generated a variety of different reactions and "new narratives about the Roman Empire in diverse contexts".