Roman legions under Pompey conquered much of the Levant in 63 BC, inaugurating a period of Roman rule that lasted four centuries. In 106 AD, Emperor Trajan annexed Nabataea unopposed, and rebuilt the King’s Highway which became known as the Via Traiana Nova road. The Romans gave the Greek cities of Transjordan–Philadelphia (Amman), Gerasa (Jerash), Gedara (Umm Qays), Pella (Tabaqat Fahl) and Arbila (Irbid)–and other Hellenistic cities in Palestine and southern Syria, a level of autonomy by forming the Decapolis, a ten-city league. Jerash is one of the best preserved Roman cities in the East; it was even visited by Emperor Hadrian during his journey to Palestine.
In 324 AD, the Roman Empire split, and the Eastern Roman Empire–later known as the Byzantine Empire–continued to control or influence the region until 636 AD. Christianity had become legal within the empire in 313 AD after Emperor Constantine converted to Christianity. The Edict of Thessalonka made Christianity the official state religion in 380 AD. Transjordan prospered during the Byzantine era, and Christian churches were built everywhere. The Aqaba Church in Ayla was built during this era, it is considered to be the world’s first purpose built Christian church. Umm ar-Rasas in southern Amman contains at least 16 Byzantine churches. Meanwhile, Petra’s importance declined as sea trade routes emerged, and after a 363 earthquake destroyed many structures, it declined further, eventually being abandoned. The Sassanian Empire in the east became the Byzantines’ rivals, and frequent confrontations sometimes led to the Sassanids controlling some parts of the region, including Transjordan.
In 629 AD, during the Battle of Mu’tah in what is today Al-Karak, the Byzantines and their Arab Christian clients, the Ghassanids, staved off an attack by a Muslim Rashidun force that marched northwards towards the Levant from the Hejaz (in modern-day Saudi Arabia). The Byzantines however were defeated by the Muslims in 636 AD at the decisive Battle of Yarmouk just north of Transjordan. Transjordan was an essential territory for the conquest of Damascus. The first, or Rashidun, caliphate was followed by that of the Ummayads (661–750). Under the Umayyad Caliphate, several desert castles were constructed in Transjordan, including: Qasr Al-Mshatta and Qasr Al-Hallabat.
The Abbasid Caliphate’s campaign to take over the Umayyad’s began in Transjordan. A powerful 749 AD earthquake is thought to have contributed to the Umayyads defeat to the Abbasids, who moved the caliphate’s capital from Damascus to Baghdad. During Abbasid rule (750–969), several Arab tribes moved northwards and settled in the Levant. As had happened during the Roman era, growth of maritime trade diminished Transjordan’s central position, and the area became increasingly impoverished. After the decline of the Abbasids, Transjordan was ruled by the Fatimid Caliphate (969–1070), then by the Crusader Kingdom of Jerusalem (1115–1187).
The Crusaders constructed several Crusader castles as part of the Lordship of Oultrejordain, including those of Montreal and Al-Karak. The Ayyubids built the Ajloun Castle and rebuilt older castles, to be used as military outposts against the Crusaders. During the Battle of Hattin (1187) near Lake Tiberias just north of Transjordan, the Crusaders lost to Saladin, the founder of the Ayyubid dynasty (1187–1260). Villages in Transjordan under the Ayyubids became important stops for Muslim pilgrims going to Mecca who travelled along the route that connected Syria to the Hejaz. Several of the Ayyubid castles were used and expanded by the Mamluks (1260–1516), who divided Transjordan between the provinces of Karak and Damascus. During the next century Transjordan experienced Mongol attacks, but the Mongols were ultimately repelled by the Mamluks after the Battle of Ain Jalut (1260).