In the late 4th century BC, Alexander the Great conquered the region, putting it under Hellenistic Seleucid rule for over two centuries. The Seleucids introduced the Indo-Anatolian and Greek term "Syria" to the region. This name had for many centuries been the Indo-European word for "Assyria" and specifically and only meant Assyria; however, the Seleucids also applied it to The Levant (Aramea, causing both the Assyria and the Assyrians of Iraq and the Arameans and The Levant to be called Syria and Syrians/Syriacs in the Greco-Roman world. The Parthians (247 BC – 224 AD) from Persia conquered the region during the reign of Mithridates I of Parthia (r. 171–138 BC). From Syria, the Romans invaded western parts of the region several times, briefly founding "Assyria Provincia" in Assyria. Christianity began to take hold in Iraq (particularly in Assyria) between the 1st and 3rd centuries, and Assyria became a centre of Syriac Christianity, the Church of the East and Syriac literature. A number of independent states evolved in the north during the Parthian era, such as Adiabene, Assur, Osroene and Hatra. The Sassanids of Persia under Ardashir I destroyed the Parthian Empire and conquered the region in 224 AD. During the 240s and 250's AD, the Sassanids gradually conquered the independent states, culminating with Assur in 256 AD. The region was thus a province of the Sassanid Empire for over four centuries, and became the frontier and battle ground between the Sassanid Empire and Byzantine Empire, with both empires weakening each other, paving the way for the Arab-Muslim conquest of Persia in the mid-7th century. The Arab Islamic conquest in the mid-7th century AD established Islam in Iraq and saw a large influx of Arabs. Under the Rashidun Caliphate, the prophet Muhammad's cousin and son-in-law, Ali, moved his capital to Kufa when he became the fourth caliph. The Umayyad Caliphate ruled the province of Iraq from Damascus in the 7th century. (However, eventually there was a separate, independent Caliphate of Córdoba in Iberia.) The Abbasid Caliphate built the city of Baghdad along the Tigris in the 8th century as its capital, and the city became the leading metropolis of the Arab and Muslim world for five centuries.