After a generation of confusion, known as the "dark age" of the Parthian Empire, unity was restored by the Arsacid king Phraates III (r.70/69-58/57). Assassinated in 58/57, he was succeeded by his sons Mithradates IV, who was in control of the western part of the kingdom, and Orodes II, who controled the east. The two brothers soon started to quarrel.
Orodes forced Mithradates to flee to the west, across the river Euphrates, to the Roman Republic, but with Roman support, he managed to return. In 55 BCE, he was again recognized in Babylon, and in the next year he minted coins in Seleucia on the Tigris. However, his brother Orodes had already sent an army, commanded by the Surena, who defeated Mithradates and captured him. The prisoner-king was later executed.
The Romans had promised support to Mithradates and indeed, they sent an army, commanded by Crassus, one of the triumvirs (with Pompey the Great and Julius Caesar). He had secured his flank by concluding an alliance with king Artavasdes II of Armenia, crossed the Euphrates, and marched on Edessa, but Surena defeated him in the marshes of the river Balikh near Carrhae (53 BCE). At the same time, Orodes attacked Armenia and forced its king Artavasdes II to switch sides. Armenia would, from this moment on, be a permanent bone of contention between the Romans and the Parthians.
In 52 BCE, Surena invaded the Roman province of Syria, but he achieved little and was killed on his return: he had become too powerful. Although there were, from now on, no armies active in the regions, the rensions with Rome remained. Several years later, in c.50 BCE, the Romans supported a rival Arsacid king, prince Pacorus, but he made his peace with Orodes. Still, it was a sign of the times.
A few years later, in 41, when the Romans were involved in a civil war between the adherents and the murderers of Julius Caesar, Orodes made Pacorus his co-ruler and sent him again to the west. The Parthian armies penetrated deep into Roman territory and made the Jewish high priest, the Hasmonaean Hyrcanus II, prisoner, but in the end, the Romans restored order and Pacorus was killed in action.
Orodes now appointed his son Phraates IV as successor; the crown prince did not wait for his benefactor to die but immediately had him killed.
The chronology of the Arsacid kings of the Parthian Empire is less well-understood than, for example, the sequence of Seleucid and Ptolemaic kings or the emperors of Rome. This information is based on the researches by G.R.F. Assar, as published in "Iran under the Arsakids, 247 BC – AD 224/227" in: Numismatic Art of Persia (2011).