The area of the kingdom was perhaps roughly coterminous with that of the Roman province of Osrhoene. The great loop of the Euphrates was a natural frontier to the north and west. In the south
Batnae was capital of the semi-autonomous principality of
Anthemusias until its annexation by Rome, in AD 115. The eastern boundary is uncertain; it may have extended to Nisibis or even to Adiabene in the first century AD. Ḥarrān, however, only 40 km south of Edessa, always maintained its independent status as a Roman colonia. Edessa, the capital of the ancient kingdom, was a fortress of considerable strength and a staging post both large and nearest to the Euphrates. It was an important road junction; an ancient highway, along which caravans carried merchandise from China and India to the West, meeting there a north–south road connecting the Armenian Highlands with Antioch. Inevitably, Edessa figured prominently on the international stage. In 64 BC, as
Pompey waged war on the
Parthian Empire,
Abgar II of Osrhoene had sided with the Romans when
Lucius Afranius occupied
Upper Mesopotamia. The king was initially an ally of the Roman general
Marcus Licinius Crassus in his campaign against the Parthians in 53 BC, but Roman historians allege that he betrayed Crassus by leading him to deviate from his safe route along the river and instead into an open desert, where the troops suffered from the barrenness and thus were vulnerable to cavalry attack. Abgar is said to have met with
Surenas, the Parthian general, and informed him of the Roman movements. The enormous and infamous
Battle of Carrhae followed and destroyed the entire Roman army. Just prior to the battle, Abgar made a pretext to ride away. However, modern historians have questioned whether Abgar intended to betray the Romans and instead may have simply been leading them along an old Arab trade route. According to a Syriac source, Abgar died later that year. In the early 2nd century AD, King
Abgar VII joined the Emperor
Trajan's campaign into Mesopotamia and entertained him at court. The king later rebelled against the Romans, however, which led to the Roman general
Lucius Quietus sacking Edessa and putting an end to Osrhoene's independence in 116. In 123, during the reign of
Hadrian, the Abgarid dynasty was restored with the installation of Ma'nu VII, and Osroene was established as a client kingdom of the Empire. After the
Roman–Parthian War of 161–166 under
Marcus Aurelius, forts were built and a Roman
garrison was stationed in
Nisibis. In 195, following a civil war in which the kingdom had supported his rival
Pescennius Niger,
Septimius Severus mounted an invasion and annexed the territory as a new province, making Nisibis the capital. However, the emperor did allow the king, Abgar XI, to retain the city of Edessa and a small territory surrounding it. In 213, the reigning king was deposed by
Caracalla, and the remaining territory was incorporated into the Roman province of Osroene. According to legends (without historical justification), by 201 AD or earlier, under King Abgar the Great, Osroene became the first Christian state. It is believed that the
Gospel of Thomas emanated from Edessa around 140. Prominent early Christian figures have lived in and emerged from the region such as
Tatian the Assyrian, who came to Edessa from Hadiab (
Adiabene). He made a trip to Rome and returned to Edessa around 172–173. Tatian was the editor of the
Diatessaron, which was the primary sacred text of
Syriac-speaking Christianity until in the 5th century the
bishops
Rabbula and
Theodoret suppressed it and substituted a revision of the Old Syriac Canonical Gospels (as in the
Syriac Sinaiticus and
Curetonian Gospels). Then, Edessa was again brought under Roman control by
Decius and it was made a center of Roman operations against the
Sasanian Empire.
Amru, possibly a descendant of Abgar, is mentioned as king in the
Paikuli inscription, recording the victory of
Narseh in the Sassanid civil war of 293. Historians identify that Amru as
Amru ibn Adi, the fourth king of the
Lakhmids, which was then still based in
Harran, not yet moved to
al-Hirah in southern Mesopotamia. Many centuries later,
Dagalaiphus and
Secundinus duke of Osrhoene, accompanied
Julian in his war against the Sasanian emperor,
Shapur II, in the 4th century. ==Roman province==