Why did the Romans not finish off the Persian empires when they repeatedly captured the capital city Ctesaphion and even took some royals hostage. Why did they not press on further and finish of the Iranian kingdoms like the Arabs did later. Why did they allow the Persians to survive when they could have delivered a knockout blow. It seems like both Persians and Byzantines were interested in their peripheries of their empire rather than take each others heartlands. It seems like they were enemies only upto a certain point and did not want to extinguish each other. Why else would this happen?