Parthian decentralization of government allowed the continutation of soveriegn governance despite the capture of the Parthian capital at Cstephon by Trajan and Lucius Varus. Also the natural borders of the Empire were long established at the banks of the Rhine, Danube, and Euphrates and there was not much effort to expand those borders after Trajans campaigns (Julian the Apostate and his war against Shapur being the major exception although they were the Sassinids by that time). The Parthians also did not do much to disrupt trade from the Far East and therefore although they limited Roman expansion they did not seriously try to conquer Romans lands. Finally the Roman-Parthian standoff was mutual in that the Parthians could probably not hope to inflict any serious damage to the Roman Empire and when properly equipped, ambitious, and led the Roman Legions where capable of defeating the Parthians.