You should note that Persia eventually did intervene in both Yemen and the Central Asian plains, although in the latter area their involvement was certainly more on the defensive. Axum was certainly quite an advanced nation with close contact with the Romans, but perhaps the jungles and forest of Nubia held little allure to the emperors despite the rich cities that lay beyond.

In my opinion, Rome and Persia duke it out for over so long because each of them was the others's greatest prize. Rome held Asia Minor, Syria and Egypt while Persia held Mesopotamia, and outside of China and India these were the most economically and culturally developed regions of the world at that time. There's little to gain other than glory and retribution (which the Romans were admittedly fools for) during a campaign in Caledonia or Germania for the Romans, and the Persians always fought defensively against the Central Asian Turks. Both empires also have the "burden of history" on them. The Roman emperors, fancying themselves the heirs of Alexander the Great, fantasized of once again conquering all the lands to the borders of India, while every new Shanshah were reminded to regain the glory of their Achaemenid predecessors and once again ruled from the Mediterranian to the Indus.

Persian(Iranian) ward of Roman attacks for 1000 years and they

never could get Iran.

It is also interesting to note that it appears the children of Rome (a.k.a. Western Civilization), find themselves in conflict with this troubled and endlessly fought over region to this day.

Thank you Mr. Strauss if you're still following this forum in 2012.