Osroene (Roman province)

Osroene (), also spelled Osrohene and Osrhoene, was a Roman province which existed for nearly 400 years. It was formed after the absorption of the Kingdom of Osroene in 244 CE and served as a frontier province against the Sassanid Empire until the Muslim conquests of the 7th century.
For the whole of its existence, the province would remain a bone of contention between the Romans and their eastern neighbors, the Sassanid Persians, suffering heavily in the recurrent Roman–Persian Wars. War broke out after the death of the Roman emperor Decius in 251 and the province was invaded by the Persians. In the second half of the 250s, the Persian shah Shapur I (r. ca. 240–270) attacked the Roman east, which was defended by the Roman emperor Valerianus (r. 253–260), whom he captured at Edessa in 260. In the next year however, Shapur was heavily defeated by Odaenathus of Palmyra and driven out of Osroene and Mesopotamia.
It was taken and retaken several times. Being a province on the frontier it had a Roman legion stationed there, Legio III Parthica and its Castrum (homebase) was Resaena though there are some doubts on that fact.
Since Emperor Diocletian's Tetrarchy reforms during his reign 284-305 CE, it was part of the diocese of Oriens, in the praetorian prefecture of the same name. According to the late 4th-century "Notitia Dignitatum", it was headed by a governor of the rank of "praeses", and was also the seat of the "dux Mesopotamiae", who ranked as "vir spectabilis" and commanded (c. 400) the following army units:
as well as, 'on the minor roll', apparently auxiliaries: