[3][9] The tradition of Ghassanid migration finds support in the Geography of Ptolemy, which locates a tribe called the Kassanitai south of the Kinaidokolpitai and the river Baitios (probably the wadi Baysh).

[3] Their earliest appearance in records is dated to 473, when their chief, Amorkesos, signed a treaty with the Byzantine Empire acknowledging their status as foederati controlling parts of Palestine.

Geographically, it occupied much of the eastern Levant, and its authority extended via tribal alliances with other Azdi tribes all the way to the northern Hijaz as far south as Yathrib (Medina).

The Ghassanid king al-Harith ibn Jabalah (reigned 529–569) supported the Byzantines against the Sasanians and was given in 529 by the emperor Justinian I, the highest imperial title that was ever bestowed upon a foreign ruler; also the status of patricians.

The Ghassanids, who had successfully opposed the Lakhmids of al-Hirah in Lower Mesopotamia, prospered economically and engaged in much religious and public building; they also patronized the arts and at one time entertained the Arab poets al-Nabighah and Hassan ibn Thabit at their courts.

[3] The nascent Muslim state in Medina, first under the Islamic prophet Muhammad (d. 632) and lastly under the second caliph, Umar (r. 634–644), made abortive attempts to contact or win over the Ghassan of Syria.

[18] The last phylarch of the Ghassan, Jabala ibn al-Ayham, stories of whom are shrouded in legend, led his tribesmen and those of Byzantium's other allied Arab tribes in the Byzantine army that was routed by the Muslims at the Battle of Yarmouk in c. 636.

[20] Mu'awiya actively sought the militarily and administratively experienced Syrian Christians, including the Ghassanids, and members of the tribe served him and later Umayyad caliphs as governors, commanders of the shurta (select troops), scribes, and chamberlains.

Several descendants of the tribe's Tha'laba and Imru al-Qays branches are listed in the sources as Umayyad court poets, jurists, and officials in the eastern provinces of Khurasan, Adharbayjan and Armenia.

The Ghassan, along with their tribal allies in Syria, especially the Kalb, supported continued Umayyad rule to secure their interests under the dynasty, and nominated Mu'awiya's distant cousin, Marwan I, as caliph during a summit of the Syrian tribes in the old Ghassanid capital of Jabiyah.

[24] The toppling of the Umayyads and the advent of the Iraq-based Abbasid Caliphate in 750 "was disastrous for the power, wealth and status of the Arab tribes in Syria", including the Ghassan, according to the historian Hugh N. Kennedy.

By the 9th century, the tribe had adopted a settled life, being recorded by the geographer al-Ya'qubi (d. 890) to be living in the Ghouta gardens region of Damascus and in Gharandal in Transjordan.

Both were militarily successful allies of the Byzantines, especially against their enemies the Lakhmids, and secured Byzantium's southern flank and its political and commercial interests in Arabia proper.

According to the historian Warwick Ball, the Ghassanids' promotion of a simpler and more rigidly monotheistic form of Christianity in a specifically Arab context can be said to have anticipated Islam.

[30] Ghassanid rule also brought a period of considerable prosperity for the Arabs on the eastern fringes of Syria, as evidenced by a spread of urbanization and the sponsorship of several churches, monasteries and other buildings.

The surviving descriptions of the Ghassanid courts impart an image of luxury and an active cultural life, with patronage of the arts, music and especially Arab-language poetry.