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.

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.

[1] 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.

With the defeat of Andragoras, the Parthian aristocracy joined forces with the Parni to create a kingdom that was well situated for imperial expansion.

[6][7] 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.

Once Seleucus eventually reached Parthia, Arsaces withdrew into Central Asia in a calculated strategic withdrawal in the face of the Seleucid army.

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.

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.

By 210 BCE, 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.

Antiochus was forced to divide his army into several large bodies to pass the rough terrain, making his phalanx and baggage train particularly vulnerable.

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.

Arsaces II had so far failed to halt the Seleucid march, so he decided to drastically change his strategy to a siege defense.

With the defense's falling and the city being demolished, the Parthian soldiers began preparing for a final assault on the Seleucid siege lines.

As well as being stripped of most of their territory, the Parthian military had been greatly weakened after the defeats it suffered at the hands of the Seleucids and was disabled for some time.

However, with the sudden death of Antiochus III in 187 BCE, Arsaces II took advantage of Seleucid weakness and reoccupied Hecatompylus and began minting coins in his name, asserting Parthia's independence once again.

During his fifteen-year reign, Phriapatius strengthened the Parthian army and launched attacks on the eastern border of Seleucid Media.

Around this time, the king of the Seleucid Empire, Antiochus IV was suppressing the Maccabean revolt, however the rapid expansion of Phraates had drawn his attention.

Antiochus then attempted to loot the region surrounding the old Achaemenid royal city of Persepolis, but again was driven back by the local people.

During the 160s BCE, the king of the Seleucid Empire, Demetrius I, stationed generals in the east to secure and protect the eastern provinces from Parthian aggression.

In 145 BCE, Mithridates returned east to deal with a succession crisis in the Parthian vassal state of the Greco-Bactrian kingdom as well as to subdue tribes in modern-day south-eastern Iran and south-western Pakistan.

During this time, The Elymaens perceiving Parthian weakness, took the opportunity to invade Babylonia and wreak havoc on the region, burning the city of Apamea.

In 140 BCE, Demetrius II decided to pursue war with the Parthians in order to advance his standing and retake Mesopotamia, which was an important source of revenue for him.

Moreover, a victorious campaign would have given him legitimacy as the king needed to recruit troops in Syria to continue the civil war and would have boosted his prestige, wealth, and resources tremendously.

Bagasis, with a much smaller force, continuously harassed the Seleucid army in an attempt to coerce Demetrius into making mistakes while advancing deeper into Babylonia.

During this time, Bagasis intentionally made it appear as if he was losing the skirmishes in order to inspire complacency and overconfidence in Demetrius.

In order to recover this territory, Antiochus VII Sidetes, launched a counter-offensive against the Parthians in 130 BC, initially defeating them twice in battle.

Seeing an opportunity to strike, the Parthians, under Phraates II, defeated and killed Antiochus at the Battle of Ecbatana in 129 BC, and proceeded to destroy and capture the rest of his massive army, thus ending the Seleucids' attempt to retake Persia.