The Seleucid–Parthian Wars were a series of conflicts between the Seleucid Empire and the Parthian Empire which resulted in the ultimate expulsion of the Seleucids from the Iranian Plateau and the surrounding regions. The wars were caused by the mass migration of the nomadic Iranian Parni tribe into Parthia and the establishment of the Parthian state, which challenged Seleucid hegemony.
Background
In 323 BC, the Seleucid Empire was founded by Seleucus I Nicator, a general of Alexander the Great. Stretching from Syria to the Indus River and comprising most of Alexander's realm, the Seleucid state was the most powerful of the Diadochi kingdoms that sprang up after Alexander's death. Quickly however, the Seleucids ran into trouble trying to maintain such an extended realm, facing constant warfare against the other Hellenistic states in the west and with unrest amongst their Iranian peoples in the east.
Taking advantage of the Seleucids' preoccupation with the wars against a Celtic invasion of Asia Minor in the west, and the chaos of the Third Syrian War around 245 BC, Diodotus and Andragoras, the Seleucid satraps of Bactria and Parthia respectively, declared their remote provinces independent states. However, in around 238 BCE, the Parni, an Iranian tribe from the central Asian steppes under Arsaces, invaded Andragoras' domain, defeated and killed him, and took over the land.
First Parni invasion of Margiana
Sometime in the 280s BCE, the Parni attempted to invade Seleucid Margiana in order to expand their regional power and security. The Seleucids quickly dispatched Demodamas, the satrap of Bactria and Sogdiana, to neutralize the Parni threat. Demodamas launched his campaign, defeated the Parni in Margiana, and restored Seleucid rule in the region. The military capabilities of the Parni had been weakened as a result of the Seleucid retaliation and victory; however, after Demodamas restored the territorial integrity of the Seleucids in Central Asia, he ended his campaign abruptly and failed to fully neutralize the Parni.
Second Parni invasion of Margiana
The Parni had been militarily disabled for several decades as a result of Demodamas' successful campaign. Arsaces I became the king of the Parni in 248 BCE and led them on a second campaign against Seleucid Margiana in 246 BCE. Once again, a Seleucid satrap, this time Diodotus, was sent to repel the invasion. Diodotus was able to repel Arsaces I and secure Margiana. Despite the victory, the Seleucids once again failed to completely neutralize the Parni threat.
Parni conquest of Parthia
In 247 BCE, the Seleucid satrap of Parthia, Andragoras revolted and declared independence in Parthia. By the early 230s BCE, Arsaces had recovered from his setback in Margiana, and in 238 BCE, he invaded the now independent Parthia. Arsaces I was able to defeat and kill Andragoras and establish a kingdom that would become the Parthian Empire. With the defeat of Andragoras, the Parthian aristocracy joined forces with the Parni to create a kingdom that was well situated for imperial expansion. Around the same time, Arsaces I attacked and successfully seized Hyrcania, incorporating it into his kingdom. The Parni began assimilating into Parthian society, adopting the language and name of the local people, thus becoming the Parthians.
Campaign of Seleucus II Callinicus
Arsaces I had been successful in establishing a kingdom in Parthia; however, the Parthians still had limited resources and could expect an imminent Seleucid retaliation. Knowing this, Arsaces began levying soldiers and expanding his territory. With the Seleucid east disintegrating, a campaign to reconquer the region, secure the eastern frontier, and restore Seleucid prestige was increasingly necessary. After the death of his father, Antiochus II, in 246 BCE, Seleucus II succeeded him as ruler but was unable to respond to the Parthian threat because of an ongoing civil war with his brother, Antiochus Hierax, in the west.
Eventually, the two brothers reached a truce in 236 BCE, and Seleucus was able to begin an expedition against the Parthians in 235 BCE. Seleucus began his campaign by settling affairs in Babylonia and Media before marching on Parthia. Once Seleucus eventually reached Parthia, Arsaces withdrew into Central Asia in a calculated strategic withdrawal in the face of the Seleucid army. Arsaces' withdrawal was not a retreat or disorganized rout but rather part of a predetermined strategy to inspire overconfidence and complacency in Seleucus. Arsaces remained the master of the situation. Shortly after their withdrawal, the Parthians inflicted a major defeat on the Seleucid army, possibly even capturing Seleucus. Whatever the case may be, the Seleucids were decisively defeated, as recorded by Justin:
"and not long after, engaging with king Seleucus, who came to take vengeance on the revolters, he obtained a victory; and the Parthians observe the day on which it was gained with great solemnity, as the date of the commencement of their liberty."
— Justin, Book XVI, 41.4.9
Ammianus similarly explains thus:
After many glorious and valiant deeds, and after he [Arsaces I] had conquered Seleucus [I] Nicator [in reality, Seleucus II], successor of the said Alexander [the Great], on whom his many victories had conferred that surname, and [after Arsaces] had driven out the Macedonian [that is,
the Seleucid] garrisons [from Parthia], he passed his life in quiet peace, and was
a mild ruler and judge of his subjects."
— Ammianus, 23.6.3
Whether Seleucus was captured or not, he eventually was able to return west and resume the civil war against Antiochus Hierax. It appears that Seleucus was forced to acquiesce to Arsaces and leave Parthia to its devices. Despite their recent victory, Parthian control remained fragile.
Campaign of Antiochus III
The campaign of Seleucus II had ended with the ultimate defeat of the Seleucids, but later Seleucid kings had ambitions to restore their hegemony in the east. Antiochus III the Great became king of the Seleucid Empire after his father, Seleucus II, had died in 225 BC and his brother, Seleucus III, had been assassinated in 222 BC. Antiochus faced several challenges during the initial years of his reign. He was met with the challenge of suppressing a revolt by the powerful satrap Molon, and had fought Ptolemy IV to a stalemate in the Fourth Syrian War. After matters in the west were settled, Antiochus began preparing for a campaign in the east of his empire to reconquer the newly independent states of Parthia and Bactria. There were several personal and geopolitical reasons for Antiochus' eastern campaign, namely to punish the Parthians and Bactrians, restore Seleucid hegemony in the east, and avenge his father, Seleucus II. Antiochus spent a year in Babylonia organizing his army and logistics. Antiochus had raised an army of 70,000, then set out and began marching in the spring of 209 BCE. By 210 BC, the Seleucids had entered Media and looted 4000 talents of gold and silver from the Temple of Aene in Ecbatana in order to fund their campaign. Before invading Parthia proper, Antiochus settled affairs in other parts of the empire, bringing Armenia, Media, Atropatene, Elymais, and Persia back into the Seleucid fold.
Situation in Parthia
By the time of the invasion of Antiochus III, Arsaces I had died, and the throne passed to his son, Arsaces II; however, he lacked the political and military savvy of his father. Arsaces had a tremendous military disadvantage and from the beginning of the Seleucid invasion, miscalculated and misjudged their objectives. Arsaces continued to make strategic errors, which led to his ultimate defeat.
The Campaign Against Parthia
The invasion of Parthia began with Antiochus III outmaneuvering Arsaces II by crossing a desert in western Parthia adjacent to the regions of Choarene and Komisene, which is where Arsaces had stationed his army and wrongly assumed the Seleucid army would attack from. As Arsaces realized this mistake, he attempted to rectify it by sending detachments of his army to destroy water wells in the area to slow the massive Seleucid army. Antiochus, in response, sent a general by the name of Nicomedes with a force of 1,000 cavalry to secure the wells. The Parthian units lacked the necessary strength to adequately fight the Seleucid force under Nicomedes and therefore implemented a feigned retreat to lure Nicomedes into giving chase towards the main Parthian army where it could be dealt with. Nicomedes refused to fall for the trap and did not pursue the Parthian detachments, and successfully secured the wells and drove away the Parthians. This was another major blunder on Arsaces' behalf. Arsaces had sent too few men to destroy the wells and moved too far away to support his units in need. Arsaces decided to abandon his capital, Hecatompylus, and retreat into the mountains of Hyrcania because he recognized that his army was not able to match the enormous Seleucid army. Antiochus had expected to face Arsaces in a conventional battle at Hecatompylus and wondered why Arsaces had abandoned it. Asraces' unexpected withdrawal had complicated the situation for Antiochus, who had hoped to deal with Parthian affairs and march against Bactria.
Arsaces had hoped to gain the initiative and overwhelm the Seleucid army in the mountainous region of Hyrcania. Unfortunately for the Parthians, the rugged terrain nullified the traditional advantages of a Parthian army, namely mobility and speed. The Parthian army was forced to begin dismounting cavalry and acting as infantry. Antiochus wasted no time and began pursuing the Parthian army by crossing the Alborz into Hyrcania. The Seleucid army was slowed down by obstacles created by the Parthians. Antiochus was forced to divide his army into several large bodies to pass the rough terrain, making his phalanx and baggage train particularly vulnerable. Arsaces saw an opportunity to recover from his previous mistakes by striking the Seleucid army while it was vulnerable, and so the Parthian army began harassing the slow Seleucid army. Antiochus decided to cover his army with bands of lightly armed and mobile units to protect it from Parthian attacks. Despite being dug into ambush positions, the isolated Parthian units were flushed out by the lightly armed Seleucid detachments. This represents another failure on Arsaces' behalf. Seeing the ineffectiveness of his soldiers in the mountain skirmishes, Arsaces decided to withdraw his army and regroup at the summit of a mountain pass.
Battle of Mount Labus
Arsaces II chose to make a stand at Mount Labus because the terrain allowed him to implement his cavalry. The Parthian and Seleucid armies met and fought for an entire day. Arsaces' objective was to split the Seleucid phalanx from the main body of the Seleucid army so that it could be destroyed, defeating the Seleucid army in detail. However, Antiochus III prevented his men from pursuing the Parthians, who employed offensive hit-and-run tactics throughout the battle in hopes of inspiring the phalanx to give chase. Ultimately, the Parthian cavalry was unable to break the Seleucid phalanx, and the phalanx was unable to advance, putting itself in a vulnerable situation that the Parthians could exploit, creating a stalemate. The night after the battle, contingents of the Seleucid army flanked the Parthian army, causing the Parthians to retreat further. After this victory, Antiochus resumed his march and captured the unprotected city of Tambrax, which contained a royal palace.