Why Did the Parthian Empire Fall?

The Parthian Empire fell in 224 CE when the Sasanian king Ardashir I defeated and killed the last Arsacid king of kings, Artabanus IV, at the Battle of Hormizdagan. The fall was the culmination of a long process of internal decline, foreign pressure, and dynastic crisis that had been weakening the Parthian state for more than a century. The proximate cause was the Sasanian revolution; the underlying causes were the structural weaknesses of the Arsacid monarchy itself.

Parvaneh Pourshariati’s Decline and Fall of the Sasanian Empire (2008) is the indispensable modern treatment; A. D. H. Bivar’s Cambridge History of Iran chapter (1983) is the standard account of the political history; M. Rahim Shayegan’s Arsacids and Sasanians (2011) is the best treatment of the political-ideological dimension.

The structural problem

The Parthian state was, throughout its history, a coalition. The king of kings ruled with the consent and active support of the great noble families — the Suren, the Karen, the Ispahbudhan, the Aspahapet, the Spandiyadh, the Sihbuya, and the Zhinvat — and of the Zoroastrian priesthood. The coalition held together when the royal centre was strong. It fell apart when the royal centre was weak, and the Parthian monarchy of the second and third centuries was, on the whole, weak. The Arsacid kings of the second century were, with the exception of Vologases I (r. c. 51–80 CE), mostly short-reigning and mostly deposed or killed by the great houses. The succession was disputed; the great families maintained their own cataphract units and their own foreign policies; and the king of kings had, in many cases, less power than the Suren-Pahlav who crowned him.

The Pourshariati thesis, that the great noble families were the dominant political actors of the late Arsacid state, and that the Sasanian revolution was essentially a revolt of one of these families (the Sasanians of Persis) against the central authority, is the most useful framework for understanding the fall. The thesis is contested — Touraj Daryaee, in Sasanian Persia (2009), reads the Sasanian revolution as a more popular, more nationalist, and more Zoroastrian event — but the underlying observation, that the Arsacid state was structurally vulnerable to a revolt from the Iranian heartland, is not in dispute.

The foreign pressure

The Parthian state was also weakened by the long wars with Rome. The wars of Trajan (114–117 CE) and Septimius Severus (194–202 CE) were particularly destructive. Trajan conquered Mesopotamia and Ctesiphon in 116, and the Roman occupation was a major shock to the Parthian system, even though it was abandoned within two years of Trajan’s death. Septimius Severus invaded twice, in 194–195 and 202, and on the second occasion he annexed the Roman province of Mesopotamia and built a line of fortifications along the Khabur and the upper Tigris. The Roman presence in northern Mesopotamia was now intended to be permanent, and the Parthian state had to accept it.

The wars with Rome forced the Parthian kings to make concessions to the great noble families in order to raise armies. The concessions strengthened the very families whose political power was undermining the monarchy. The pattern is the classic one of a state in fiscal-military crisis: the central authority has to bargain with the provincial magnates for revenue and troops, and the bargain strengthens the magnates at the expense of the centre. The Arsacid kings of the second century were, by that logic, the victims of a long-term structural failure that no individual king could reverse.

The secession of the peripheries

The Parthian state was further weakened by the secession of subordinate kingdoms in the Caucasus and the Iranian heartland. The kingdom of Armenia, which had been ruled by a branch of the Arsacid dynasty, became increasingly independent in the second and third centuries. The Caucasian kingdoms of Iberia, Albania, and Colchis all asserted their independence. The secession of these subordinate kingdoms reduced the Parthian state to the Iranian heartland and Mesopotamia, and the loss of the Caucasian territories was a major strategic blow. The Roman annexation of Armenia, formalised under Trajan, was the end of an era.

The Sasanian revolution

The Sasanian revolution, which culminated in the defeat of Artabanus IV at the Battle of Hormizdagan in 224 CE, was the central event in the fall of the Parthian state. The Sasanian dynasty, which took its name from Sasan, the grandfather of Ardashir I, was a local dynasty of Persis, the Iranian heartland, that had been growing in power since the early third century. The Sasanian kings, who claimed direct descent from the Achaemenids, were determined to overthrow the Arsacid dynasty and to restore the Achaemenid imperial idea.

The Sasanian revolution was led by Ardashir I, a local king of Persis who had been expanding his power at the expense of the surrounding principalities since about 220 CE. Ardashir had absorbed the kingdom of Elymais, the cities of the Diyala region, and other principalities along the Mesopotamian border, and he had begun to intervene in the affairs of the declining Arsacid empire. The decisive confrontation came in 224 CE at the Battle of Hormizdagan, where Ardashir defeated and killed Artabanus IV and proclaimed himself successor to the Achaemenids. The Arsacid dynasty was exterminated, and the Sasanian dynasty took its place.

The verdict

The fall of the Parthian state was the result of a long process of internal decline, foreign pressure, and dynastic crisis. The Parthian state, which had been the dominant power of the Near East for nearly three centuries, was weakened by internal civil wars, by the long and costly wars with Rome, and by the secession of subordinate kingdoms. The Sasanian revolution was the final blow, and the Arsacid dynasty was replaced by the more centralised, more militarily aggressive, and more self-consciously Zoroastrian Sasanian state.

The Sasanian revolution was, in one sense, a restoration. The Sasanian kings claimed descent from the Achaemenids, and they modelled themselves on the Achaemenid imperial idea. They adopted the Achaemenid title of King of Kings, restored the Achaemenid sacred fires, and revived the Achaemenid tradition of rock-cut royal tombs at Naqsh-e Rostam. The Sasanian state was thus a new beginning, but a beginning that claimed to be a return. The Sasanian revolution was, in another sense, a transformation. The Sasanian kings were more centralised, more militarily aggressive, and more self-consciously Zoroastrian than the Arsacid kings. The Sasanian state was a new thing under the sun, and the Sasanian period is a formative era in the long history of pre-Islamic Iran.

The fall of the Parthian state was a major turning point in the history of pre-Islamic Iran, and it set the stage for the great Sasanian state that would rule the Near East for more than four centuries. The Sasanian state, like the Arsacid state before it, would eventually fall. The pattern is the recurring one of pre-modern Iranian history: a state that was structurally vulnerable to internal decay, sustained for a long time by a strong royal centre, and then broken by the combination of internal crisis and external pressure.