Late antiquity is a periodization used by historians to describe the time of transition from classical antiquity to the Middle Ages in Europe and adjacent areas bordering the Mediterranean Basin. The popularization of this periodization in English has generally been credited to historian Peter Brown, after the publication of his seminal work The World of Late Antiquity (1971). Precise boundaries for the period are a continuing matter of debate, but Brown proposes a period between the 3rd and 8th centuries AD. Generally, it can be thought of as from the end of the Roman Empire's Crisis of the Third Century (235–284) to the early Muslim conquests (622–750), or as roughly contemporary with the Sasanian Empire (224–651). In the West its end was earlier, with the start of the Early Middle Ages typically placed in the 6th century, or earlier on the edges of the Western Roman Empire.
The Roman Empire underwent considerable social, cultural and organizational changes starting with the reign of Diocletian, who began the custom of splitting the Empire into Eastern and Western portions ruled by multiple emperors simultaneously. The Sasanian Empire supplanted the Parthian Empire and began a new phase of the Roman–Persian Wars, the Roman–Sasanian Wars. The divisions between the Greek East and Latin West became more pronounced. The Diocletianic Persecution of Christians in the early 4th century was ended by Galerius and under Constantine the Great, Christianity was made legal in the Empire. The 4th century Christianization of the Roman Empire was extended by the conversions of Tiridates the Great of Armenia, Mirian III of Iberia and Ezana of Axum, who later invaded and ended the Kingdom of Kush. During the late 4th century reign of Theodosius I, Nicene Christianity was proclaimed the state church of the Roman Empire.
Constantinople became the permanent imperial residence in the East by the 5th century and superseded Rome as the largest city in the Late Roman Empire and the Mediterranean Basin. The longest Roman aqueduct system, the 250 km (160 mi)-long Aqueduct of Valens was constructed to supply it with water, and the tallest Roman triumphal columns were erected there.
Migrations of Germanic, Hunnic, and Slavic tribes disrupted Roman rule from the late 4th century onwards, culminating first in the Sack of Rome by the Visigoths in 410 and subsequent Sack of Rome by the Vandals in 455, part of the eventual collapse of the Empire in the West itself by 476. The Western Empire was replaced by the so-called barbarian kingdoms, with the Arian Christian Ostrogothic Kingdom ruling Rome from Ravenna. The resultant cultural fusion of Greco-Roman, Germanic, and Christian traditions formed the foundations of the subsequent culture of Europe.
In the 6th century, Roman imperial rule continued in the East, and the Byzantine-Sasanian wars continued. The campaigns of Justinian the Great led to the fall of the Ostrogothic and Vandal Kingdoms, and their reincorporation into the Empire, when the city of Rome and much of Italy and North Africa returned to imperial control. Though most of Italy was soon part of the Kingdom of the Lombards, the Roman Exarchate of Ravenna endured, ensuring the so called Byzantine Papacy. Justinian constructed the Hagia Sophia, a great example of Byzantine architecture, and the first outbreak of the centuries-long first plague pandemic took place. At Ctesiphon, the Sasanians completed the Taq Kasra, the colossal iwan of which is the largest single-span vault of unreinforced brickwork in the world and the triumph of Sasanian architecture.
The middle of the 6th century was characterized by extreme climate events (the volcanic winter of 535-536 and the Late Antique Little Ice Age) and a disastrous pandemic (Plague of Justinian in 541). The effects of these events in the social and political life are still under discussion.
In the 7th century the disastrous Byzantine–Sasanian War of 602–628 and the campaigns of Khosrow II and Heraclius facilitated the emergence of Islam in the Arabian Peninsula during the lifetime of Muhammad. Subsequent Muslim conquest of the Levant and Persia overthrew the Sasanian Empire and permanently wrested two thirds of the Eastern Roman Empire's territory from Roman control, forming the Rashidun Caliphate.
The Byzantine Empire under the Heraclian dynasty began the middle Byzantine period, and together with the establishment of the later 7th century Umayyad Caliphate, generally marks the end of late antiquity.