 The eastern borders changed many times, as the Roman Empire was facing two major powers, The Parthian Empire and the Sasanian Empire.The Parthians were a group of Iranian peoples ruling most of Greater Iran that is in modern-day Iran, western Iraq, Armenia and the Caucasus.The Sasanians succeeded the Parthians in 224–226 and were recognised as one of the leading world powers alongside its neighbouring arch-rival the Roman (Byzantine) Empire for a period of more than 400 years.