The wars between these two all ended in some sort of stalemate, without a clear victor and always leaving the other side mostly unconquered. Can one side completely conquer the other? IE can the Persians conquer Constantinople, or the Romans Persia? Which side is more likely to win? In the long run, will complete conquest benefit the conqueror or would it just make it harder for them to rule such a vast empire, ie would it exhaust the conqueror's resources? And when Islam rises, will it be harder or easier for the Caliphs to face a large but possibly-overstretched empire?