Seleucid–Parthian wars

The Seleucid–Parthian wars were a series of conflicts between the Seleucid Empire and Parthia which resulted in the ultimate expulsion of the Seleucids from Persia and the establishment of the Parthian Empire. The wars were caused by Iranian tribes migrating into Central Asia and the inability of the Seleucids to properly defend or hold together their vast empire.

Background
In 323 BCE, the Seleucid Empire was founded by Seleucus I, 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, around 245 BC, Diodotus and Andragoras, the Seleucid satraps of Bactria and Parthia respectively, declared their remote provinces independent states. However in 238, the Parni, an Iranian tribe from the central Asian steppes under Arsaces, invaded Andragoras' domain, defeated and killed him, and took over the land. The Parni then became known as the Parthians taking their name from the Seleucid province that they had conquered. they then began to try and conquer as much of the eastern Seleucid empire as possible. they were joined in this by the now independent province of Bactria. the Seleucid king Antiochus II Theos was at the time too busy fighting a war against Ptolemaic Egypt and so the Seleucids lost most of their territory east of Persia and Media.

Campaigns of Antiochus III
Antiochus III was an ambitious Seleucid king who had a vision of reuniting Alexander the Great's empire under the Seleucid dynasty. In 209 BCE, he launched a campaign to regain control of the eastern provinces, and after defeating the Parthians in battle, he successfully regained control over the region. The Parthians were forced to accept vassal status and now only controlled the land conforming to the former Seleucid province of Parthia. However, Parthia's vassal was only nominal at best and only because the Seleucid army was on their doorstep. For his retaking of the eastern provinces and establishing the Seleuicd borders as far east as they had been under Seleucus I Nicator, Antiochus was awarded the title great by his nobles. Luckily for the Parthians, the Seleucid Empire had many enemies, and it was not long before Antiochus led his forces west to fight Ptolemaic Egypt and the rising Roman Republic.

Rise of the Arsacids and the end of the Seleucid Empire
Seleucid power began to weaken after the defeat of Antiochus at the hands of the Romans at the Battle of Magnesia effectively broke Seleucid power and in particular the Seleucid army. The Arsacids took power in Parthia and declared their full independence from the Seleucid Empire. In 148 BCE the Parthian king Mithridates I of Parthia invaded Media which was already in revolt against the Seleucid empire. After winning a decisive battle at the Battle of Ecbatana Mithridates was able to annex Media from the Seleucid Empire, and in 141 BCE they captured the major Seleucid city of Seleucia (which was the eastern capital of the Seleucid empire) which gave Mithridates control over Mesopotamia and Babylonia. These victories gave Parthia control over all the land between the Euphrates and Indus Rivers. In 139 BCE the Parthians defeated a major Seleucid counterattack and broke the Seleucid army and captured Seleucid King Demetrius II, this effectively ended Seleucid claims to any land east of the Euphrates river. In 129 BCE the Parthians drove back the last Seleucid counterattack.

The loss of so much territory sent the already enfeebled empire into a decline from which it could never recover. The Seleucid Empire became a rump state which consisted of little more than Antioch and the surrounding lands. The only reason the Seleucid empire continued to exist because the Parthians saw it as a useful buffer against the Roman Empire. When Pompey led a Roman expedition into Syria, he annexed the Seleucid Empire, and the stage was set for the Roman–Parthian Wars.

Legacy
The war brought about a lasting cultural exchange between east and west. Greek ideas would be remembered in the east long after they had been forgotten about in the west, while Persian and Indian influences in sciences, literature and architecture would be introduced to western powers such as the Romans. The westward expansion of Parthia during the war would eventually lead to clashes with the Roman Empire. The Roman–Parthian Wars would embroil these ancient empires until the 3rd century.