This paper examines the campaign of the Seleucid king Antiochus III against the Parthians (209 BCE). Following Michael I. Rostovtzeff, the author considers the main aim of this expedition Graeco-Bactria, not Parthia. He states that Antiochus III did not even try to conquer the Parthians. The king’s purpose was just to force them into making a formal peace agreement and admitting his supremacy in order to continue his march into Bactria unopposed. During this campaign Antiochus showed himself as gifted military commander capable to solve different tasks and successfully withstand all the enemy’s attempts to stop him. He had to manage his large army while crossing the desert, waging war in the mountains or storming a strong fortress. Finally the Parthians were pacificated, and their kings honoured the treaty with Antiochus at least till Mithridate I’s accession to the throne.