In 238 BC a nomadic group called the Parni, originally from Central Asia and led by a man named Arsaces settled in the satrapy (administrative district) of Parthia in northern Iran. Known as the Parthians after their successful conquest of the land, they established a line of rulers who assumed the name Arsaces as a royal title. The Seleucid king Antiochus III (reigned 223-187 BC) reconquered much of Iran around 204 BC when he campaigned in the east. However, during the 2nd century BC the Parthians extended their power across Iran and Mesopotamia. Under Mithradates I (reigned 171-139 BC) and his successors, the Parthians grew into one of the dominant powers in the Near East. They gained control of the Seleucid capital, Seleucia-on-Tigris and founded their own capital of Ctesiphon on the opposite bank.

The River Euphrates formed the Parthian-Roman border, but Rome had ambitions of dominating the Near East. Parthia’s geographical position is popularly believed to mean that it benefited from controlling the trade along the so-called Silk Road between China and Rome, yet the economic importance of this route (actually a network of routes) has been over-emphasised.