The first Arab tribes to adopt Christianity were likely Nabataeans and Ghassanids. During the fifth and sixth centuries, the Ghassanids, who at first adopted monophysitism, formed one of the most powerful confederations allied to Christian Byzantium, being a buffer against the pagan tribes of Arabia. The last king of the Lakhmids, al-Nu'man III ibn al-Mundhir, a client of the Sasanian Empire in the late sixth century, also converted to Christianity (in this case, to the Nestorian sect favoured by the native Christians of al-Hira).