Eastern armies, such as those of the Seleucid and Parthian Empires, were characterised by a mix of troops recruited from both east and west. In addition to their native conscripts, they consisted of many vassal and mercenary units. In Seleucid territories, these were used to support the limited numbers of immigrant Greek settlers that formed the mainstay of pike-armed phalanxes. Providing light infantry and cavalry, mercenaries and vassals would usually be employed to bolster the flanks. Thracian mercenaries were highly valued for their ability to fight and manoeuvre in difficult and even mountainous terrain.
