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.