Sassanid Empire in the context of Osroene (Roman province)

The Muslim conquests of Afghanistan began during the Muslim conquest of Persia as the Arab Muslims expanded eastwards to Khorasan, Sistan and Transoxiana. Fifteen years after the battle of Nahāvand in 642 AD, they controlled all Sasanian domains except in Afghanistan. Fuller Islamization was not achieved until the period between 10th and 12th centuries under Ghaznavid and Ghurid dynasties who patronized Muslim religious institutions.

Khorasan and Sistan, where Zoroastrianism was well-established, were conquered. The Arabs had begun to move towards the lands east of Persia in the 7th century. The Muslim frontier in modern Afghanistan had become stabilized after the first century of the Lunar Hijri calendar as the relative importance of the Afghan areas diminished. From historical evidence, it appears Tokharistan (Bactria) was the only area conquered by Arabs where Buddhism heavily flourished. Balkh's final conquest was undertaken by Qutayba ibn Muslim in 705.

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".

Simplicius of Cilicia (/sɪmˈplɪʃiəs/; Greek: Σιμπλίκιος ὁ Κίλιξ; c. 480 – c. 540) was a disciple of Ammonius Hermiae and Damascius, and was one of the last of the Neoplatonists. He was among the pagan philosophers persecuted by Justinian in the early 6th century, and was forced for a time to seek refuge in the Persian court, before being allowed back into the empire. He wrote extensively on the works of Aristotle. Although his writings are all commentaries on Aristotle and other authors, rather than original compositions, his intelligent and prodigious learning makes him the last great philosopher of pagan antiquity. His works have preserved much information about earlier philosophers which would have otherwise been lost.

The patriarch of the Church of the East (also known as patriarch of the East, patriarch of Babylon, the catholicos of the East (ancient Greek: καθολικός katholikós, all-encompassing [καθ- + ὅλου + -ικός]) or the grand metropolitan of the East) is the patriarch, or leader and head bishop (sometimes referred to as Catholicos or universal leader) of the Church of the East. The position dates to the early centuries of Christianity within the Sassanid Empire, and the Church has been known by a variety of names, including the Church of the East, Nestorian Church, the Persian Church, the Sassanid Church, or East Syrian.

Since 1552, rival patriarchal lines were established, traditionalist on one side and pro-Catholic on the other. In modern times, patriarchal succession is claimed from this office to the patriarchal offices of the successor churches: the Chaldean Catholic Church, the Assyrian Church of the East, and the Ancient Church of the East.

Caspiane or Kaspiane (Greek: Κασπιανή, Armenian: Կասպք Kaspkʿ) was the land populated by the tribe of Caspians, after whom it received its name. Originally a province of the Medes in the 3rd-2nd centuries BC, the land of the Caspians was conquered in the 2nd century BC, then passed to Caucasian Albania under Sassanid Persian suzerainty in the 5th century, and later became an independent state. In the 2nd century AD, it became known as Paytakaran, and after 387 AD became a part of the Caucasian Albanian larger region of Balasakan.It roughly corresponded to the modern Mugan plain and Qaradagh regions.

Jadi Rana was an Indian Hindu ruler of Sanjan, Valsad in present-day Gujarat as per the Qissa-i Sanjan, an epic poem completed in 1599, which is an account of the flight of some of the Zoroastrians who were subject to religious persecution following the fall of the Sassanid Empire, and of their early years in India, where they found refuge. A 20th-century translation of the Qissa transliterates the name as Jádi Rana.

In architecture, a squinch is a structural element used to support the base of a circular or octagonal dome that surmounts a square-plan chamber. Squinches are placed to diagonally span each of the upper internal corners (vertices) where the walls meet. Constructed from masonry, they have several forms, including a graduated series of stepped arches; a hollow, open half-cone (like half of a funnel laid horizontally); or a small half-dome niche. They are designed to evenly spread the load of a dome across the intersecting walls on which it rests, thus avoiding concentrating higher structural stress on smaller load-bearing areas. By bridging corners, they also visually transition an angular space to a round or near-circular zone.

Squinches originated in the Sassanid Empire of Ancient Persia, remaining in use across Central and West Asia into modern times. From its pre-Islamic origin, it developed into an influential structure for Islamic architecture. Georgia and Armenia also inherited the form from the Sassanids, where squinches were widely employed in buildings of all kinds. They are heavily featured in surviving or ruined medieval Christian churches of the region. An alternative approach to the structural problem of translating square space to round superstructure is the pendentive, much used in late Roman Empire and Byzantine architecture. Domes built in the Roman-influenced world utilised separately-evolved construction methods.

Aba I (or, with his Syriac honorific, Mar Aba I) or Mar Abba the Great was the Patriarch of the Church of the East at Seleucia-Ctesiphon from 540 to 552. He introduced to the church the anaphoras of Theodore of Mopsuestia and Nestorius beside the more ancient liturgical rite of Addai and Mari. Though his tenure as catholicos saw Christians in the region threatened during the Persian-Roman wars and attempts by both Sassanid Persian and Byzantine rulers to interfere with the governance of the church, his reign is reckoned a period of consolidation, and a synod he held in 544 as (despite excluding the Diocese of Merv) instrumental in unifying and strengthening the church. In 544, the Synod of Mar Aba I adopted the ordinances of the Council of Chalcedon. He is thought to have written and translated a number of religious works. After his death in February 552, the faithful carried his casket from his simple home across the Tigris to the monastery of Mar Pithyon.

Aba is a highly regarded and significantly venerated saint in the Assyrian Church of the East, the Ancient Church of the East, and the Chaldean Catholic Church. He is documented in the Ausgewählte Akten Persischer Märtyrer, and The Lesser Eastern Churches, two biographies of Eastern saints. The first seminary of the Chaldean Catholic Church outside of Iraq was established in July 2008 in El Cajon, San Diego, as the Seminary of Mar Abba the Great.

Barbaricum (from the Greek: Βαρβαρικόν, "foreign", "barbarian") is a geographical name used by historical and archaeological experts to refer to the vast area of barbarian-occupied territory that lay, in Roman times, beyond the frontiers or limes of the Roman Empire in North, Central and South Eastern Europe, the "lands lying beyond Roman administrative control but nonetheless a part of the Roman world". During the Late Antiquity, it was the Latin name for those tribal territories not occupied by Rome that lay beyond the Rhine and the Danube (but not for Persia): Ammianus Marcellinus used it, as did Eutropius. The earliest recorded mention appears to date to the early 3rd century.

In research literature, the terms 'Germania', actually Magna Germania, and 'Barbaricum' are sometimes used interchangeably, but they are not entirely identical in the chronological or the geographical senses. The extra-Roman area described as Barbaricum was, from the beginning of the Migration Period, not exclusively inhabited by Germani, even though they represented the majority of the population until the time of Late Antiquity. In the Migration Period, Alans and Huns also pushed forward into this area before and, later (from the 6th century), Slavic tribes populated the area east of the Albis (River Elbe) that the Germani had largely abandoned.