An overall survey of the long relations between the two great empires would strengthen this view. The Sasanians inherited from the Parthians a legacy of over two centuries of conflict with the western power. With a Sasanian belief in the destiny of Iran to rule over the territories once held by the Achaemenians, it was inevitable that wars between the two great powers would continue. The Sasanians might have to fight on their eastern and northern frontiers, just as the Romans had to hold the limes against the Germans and as the Byzantines sought to regain territories in Italy, Spain and elsewhere lost to Germanic kingdoms.
But the main opponent, and the only worthy opponent, of each empire was the other. Only between rulers of equal standing could proper treaties be made and affairs satisfactorily regulated. This does not mean that Persians and Romans never broke agreements or engaged in deceit towards the other party, but the Persians and the Romans regarded each other as different from the rest of the world, which was somehow barbarian. For the Persians, the Roman Caesars were the only fitting and equal counterparts of the rulers of Iran. Likewise "the king of kings" was regarded with considerable awe by the Romans. This attitude continued to the time of the Arab conquests.
The Sasanian claim to rule territories extending to the Mediterranean and Aegean seas was an overall impetus to conflict, although usually the Armenian question, conflicts of interest in Georgia and Transcaucasia, or Arab incursions in Mesopotamia would provide a casus belli. The Romans, and then the Byzantines, who were regarded as Romans by the Persians, were more on the defensive, more seeking to maintain the status quo than their opponents, although they too were not above offensive acts to extend their frontiers. By the middle of the 6th century, however, the system of defences on both sides of the Mesopotamian frontier had produced what amounted to a stalemate. Both Byzantine and Sasanian diplomacy sought to extend the area of conflict, as well as alliances, far beyond the frontiers of both states. I believe that relations of both sides with the Turks in Central Asia and with the Ethiopians and the people of southern Arabia were part of the enlargement of spheres of activity of both empires on a world stage, in one sense a forerunner of the extent of the Arab conquests. Just as in the time of the Achaemenians, at the end of the Sasanian empire ideas of the oecumene or world-state were in the air.
Some have argued that the spread of Christianity across Iran into Central Asia and to China was part of this sentiment for ecumenism and even for a universal empire, but Christians were sharply divided at the beginning of the 7th century. The Nestorian Christians of the Sasanian empire, who had broken with other Christians at the synod of Beth Lapat (Gundeshapur) in 483 and at later councils, were quarrelling with the Monophysites, just as the Monophysites in the Byzantine empire were quarrelling with the Orthodox leaders in Constantinople. Christianity had made many conversions in the Sasanian empire at the expense of Zoroastrianism, but it was far from becoming the religion of the majority as some scholars have surmised. It is true that Zoroastrianism had become stultified with too much concern for rites and rituals, and the lack of an ecumenical drive such as Christianity maintained, but it was the state church of the Sasanian empire. Unfortunately, its fate was closely bound up with the state, which accounts for its decline or stagnation after the Arab conquests.
The Sasanians did overextend themselves in the brief fulfilment of their ambitions to reconstitute the Achaemenian empire in the last years of Khusrau II, but just as the extended diplomatic activities weakened them, so did their military feats. The Persian forces were too few effectively to hold and rule Egypt, Palestine and Anatolia. Their victory proved hollow, and the subsequent events brought the prestige of the court to the lowest level in the history of the Sasanian empire. The ruler, and his court, provided the sole centre of allegiance and support for the nobility and the people. That allegiance and support were almost gone before the battle of Qadisiyya.
The story of the Arab conquest of Iran to the death of Yazdgard is clear in our now ample sources. Each province and even city had to fend for itself. Unity, a common allegiance and a common cause did not exist, and the inspired Muslim armies conquered the provinces one after the other. It was not a quick and easy conquest, for there was much fighting before the Arabs could claim the land as theirs. Once the imperial Sasanian army was crushed on the plains of Mesopotamia, however, there were no regular, trained troops to oppose the conquerors until they reached the frontiers of the Sasanian empire in the east, with the military centre of Marv, and in the north at Darband and the frontier of the Caucasus. By that time (Arabs were not settled in Marv until after 665, and they hardly reached Darband before 655) there was no incentive for the frontier troops to fight the conquerors of their homeland. The terms of peace made by the Arab armies with various cities and districts, frequently making smaller demands than the previous taxes paid to the central government of the Sasanians, induced many Persians to submit. It is clear that few subjects of the Sasanian state were concerned about the fate of the court or of the central government; local interests predominated, and the Arabs did not even have to pursue a policy of divide et impera^ for the divisions already existed. As long as the Arabs left alone local affairs, the change in masters above them had little interest for the local people.
They had no incentive to fight for a court which took little notice of them. New masters who exacted fewer taxes than the old were to be welcomed rather than fought. Such was the psychology of many Persians. When the Arabs reached the Sasanian frontier in Central Asia, they found the people no more united among themselves than the Persians, but the Sogdians and Khwarazmians had had long experience in fighting the Sasanians or Turks, and playing off one against the other. They were more proud of their local independence than the people of the Sasanian empire. It was only natural that the descendants of Yazdgard, and those who hoped for a restoration of the Sasanian empire, would turn to the east for possible assistance. Even China itself loomed as a possible haven and support for Sasanian pretenders.
The Collapse of the Empire
