Eastern kingdoms, such as the Seleucids and Parthians, relied on a formalised structure of government to administer their territories. This was modelled on the system of the Achaemenid Empire, conquered by Alexander the Great, with Persian concepts continuing to be used. In the case of the Seleucids, their centralised system eventually fragmented into separate kingdoms, like Armenia and Bactria. In contrast, the decentralised government of the Parthian Empire, in which many kingdoms paid tribute to a 'King of Kings', fared better until its replacement by the Sassanid Empire in AD224.
