Late antiquity is the time of transition from classical antiquity to the Middle Ages, generally spanning the 3rd–7th century 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. The city of Constantinople became the permanent imperial residence in the East by the 5th century and superseded Rome as the largest city in the Late Roman Empire and the Mediterranean Basin.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 -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