Osroes I of Parthia ruled the Parthian Empire

Parthia

Parthia is a region of north-eastern Iran, best known for having been the political and cultural base of the Arsacid dynasty, rulers of the Parthian Empire....

 c. 109–129. He succeeded his brother Pacorus II

Pacorus II of Parthia

Pacorus II of Parthia ruled the Parthian Empire from about 78 to 105. A son of Vonones II and brother of Vologases I, he was given the kingdom of Media Atropatene by the latter after his succession to the throne. After Vologases' death, Pacorus revolted against his brother's son and successor,...

. For the whole of his reign he contended with the rival king Vologases III

Vologases III of Parthia

Vologases III of Parthia claimed the throne of the Parthian Empire about 105, in the last days of Pacorus II of Parthia . He reigned over the eastern portion of the kingdom from 105 to 147...

 based in the east of Parthia.

Osroes invaded Armenia

Armenia

Armenia , officially the Republic of Armenia , is a landlocked mountainous country in the Caucasus region of Eurasia...

 and placed first his nephew Exedares, and then his brother Parthamasiris on the Armenian throne. This enchroachment on the traditional sphere of influence of the Roman Empire

Roman Empire

The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

— the two great empires had shared hegemony over Armenia since the time of Nero

Nero

Nero , was Roman Emperor from 54 to 68, and the last in the Julio-Claudian dynasty. Nero was adopted by his great-uncle Claudius to become his heir and successor, and succeeded to the throne in 54 following Claudius' death....

 some 50 years earlier — led to a war with the Roman emperor

Roman Emperor

The Roman emperor was the ruler of the Roman State during the imperial period . The Romans had no single term for the office although at any given time, a given title was associated with the emperor...

 Trajan

Trajan

Trajan , was Roman Emperor from 98 to 117 AD. Born into a non-patrician family in the province of Hispania Baetica, in Spain Trajan rose to prominence during the reign of emperor Domitian. Serving as a legatus legionis in Hispania Tarraconensis, in Spain, in 89 Trajan supported the emperor against...

.

In 113 Trajan invaded Parthia, marching first on Armenia. In 114 Parthamasiris surrendered and was killed. Trajan annexed Armenia to the Roman empire. Then he turned south into Parthia itself, taking the cities of Babylon

Babylon

Babylon was an Akkadian city-state of ancient Mesopotamia, the remains of which are found in present-day Al Hillah, Babil Province, Iraq, about 85 kilometers south of Baghdad...

, Seleucia

Seleucia on the Tigris

Seleucia , also known as Seleucia on the Tigris, was one of the great cities of the world during Hellenistic and Roman times. It stood in Mesopotamia, on the west bank of the Tigris River, opposite the smaller town of Ctesiphon, in present day Babil Governorate, Iraq.-Seleucid empire:Seleucia,...

 and finally the capital of Ctesiphon

Ctesiphon

Ctesiphon, the imperial capital of the Parthian Arsacids and of the Persian Sassanids, was one of the great cities of ancient Mesopotamia.The ruins of the city are located on the east bank of the Tigris, across the river from the Hellenistic city of Seleucia...

 in 116. He deposed Osroes I and put his own puppet ruler Parthamaspates

Parthamaspates of Parthia

Parthamaspates, Roman client king of Parthia and later of Osroene, was the son of the Parthian emperor Osroes I.After spending much of his life in Roman exile, he accompanied the Roman Emperor Trajan on the latter's campaign to conquer Parthia...

 on the throne. In Mesopotamia

Mesopotamia

Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

 Osroes' brother Mithridates IV

Mithridates IV of Parthia

Mithridates IV of Parthia ruled the western Parthian Empire from 129 to 140. He was the brother of Osroes I of Parthia . He was the youngest son of the Parthian King Vonones II. During the invasion of Mesopotamia by the Roman emperor Trajan in 116 he and his son Sanatruces II took up the diadem...

 and his son Sanatruces II

Sanatruces II of Parthia

Sanatruces II of Parthia, the son of Mithridates IV, was a pretender to the throne of the Parthian Empire during the disputed reign of his uncle Osroes I....

 took the diadem and fought against the Romans, but Trajan marched southward to the Persian Gulf, defeated them, and declared Mesopotamia

Mesopotamia

Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

 a new province of the empire. Later in 116, he crossed the Khuzestan mountains into Persia and captured the great city of Susa

Susa

Susa was an ancient city of the Elamite, Persian and Parthian empires of Iran. It is located in the lower Zagros Mountains about east of the Tigris River, between the Karkheh and Dez Rivers....

.

Following the death of Trajan and Roman withdrawal from the area, Osroes easily defeated Parthamaspates and reclaimed the Persian throne. Hadrian

Hadrian

Hadrian , was Roman Emperor from 117 to 138. He is best known for building Hadrian's Wall, which marked the northern limit of Roman Britain. In Rome, he re-built the Pantheon and constructed the Temple of Venus and Roma. In addition to being emperor, Hadrian was a humanist and was philhellene in...

 acknowledged this fait accompli, recognized Osroes, Parthamaspates king of Osroene

Osroene

Osroene, also spelled Osrohene and Osrhoene and sometimes known by the name of its capital city, Edessa , was a historic Syriac kingdom located in Mesopotamia, which enjoyed semi-autonomy to complete independence from the years of 132 BC to AD 244.It was a Syriac-speaking kingdom.Osroene, or...

, and returned Osroes' daughter who had been taken prisoner by Trajan.

It seems that the conflict with Rome had weakened Osroes and strengthened his rival Vologases — from about 121 there are few coins of the former and many of the latter. Osroes was succeeded by his brother Mithridates IV, who continued the struggle against Vologases.