[Amaxen]

True, I don't know enough about the reasons behind why they occasionally clashed. I think Persia had other enemies to worry about and mistakes to make, as did Byzantium. In Byzantium's case, what brought them low was their attempt to reconquer the Western Empire.

Still, the Byzantine Cataphracts were probably the best heavy cavalry in world history, except for the Persian Dhegans, whom they were a pale imitation of.

[A.J.Armitage]

True, I don't know enough about the reasons behind why they occasionally clashed.

I think it was mostly the fact that they were two big empires next to each other.

In Byzantium's case, what brought them low was their attempt to reconquer the Western Empire.

Well, it wasn't very constructive, but remember they kept going for hundreds of years after that. One thing that's interesting is that the provinces that fell to the first wave of Muslims were mostly Arian or other heretics, while the heavily orthodox areas held out until the Turks conquered Anatolia.