Antiochus III, an ambitious Seleucid king who has a vision of reuniting Alexander the Great's empire under the Seleucid dynasty, launches a campaign in 209 BCE o regain control of the eastern provinces, and after defeating the Parthians in battle, he successfully regains control over the region.

The Parthians are forced to accept vassal status and now only control the land conforming to the former Seleucid province of Parthia.

However, Parthia's vassalage is only nominal at best and only because the Seleucid army is 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 is awarded the title great by his nobles.

Luckily for the Parthians, the Seleucid Empire has many enemies, and it will not be long before Antiochus leads his forces west to fight Ptolemaic Egypt and the rising Roman Republic.