It seems to me that the Sassanids started off well against the Romans, but did less well against their Byzantine successors. I don't pretend to be a historian, but didn't the battles of Nineveh and Dara prove that the Romans, or at least their successors, could expand? After Nineveh, Heraclius gained a large amount of territory from the Sassanids, even if the Arabs swiftly overran it.