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==The future of Europe==

The Byzantine Empire was to resist Muslim attacks until it's fall in 1453. It has been described as the bulwark of Europe by many historians. If the Arabs had succeeded in capturing the city, they could have conquered the Christian Empire and used it as the gateway into Europe. At this time Eastern Europe was largely tribal and pagan, but the Byzantine Church was slowly spreading its influence, especially in the Slav and the Bulgar realms. Eventually the Byzantines secured the conversion of the peoples of the Balkans and later Russia. The Orthodox churches in Eastern Europe and Russia are the ‘daughter churches’ of the Byzantine Church <ref>Meyendorff, John. The Byzantine Legacy in the Orthodox Church (Yonkers: St Vladimir's Seminary Press, 1982), p. 19</ref>. Moreover, through the influence of the Greek Orthodox Church the culture of Constantinople was transmitted throughout Eastern Europe and Russia. This is evident in the Cyrillic Alphabet and in the art of Eastern Europe in later centuries. However, if the Arabs had been able to capture Constantinople this would almost certainly not have occurred. Instead the Caliphs armies would have eventually conquered many European territories and as in their other conquests, they would have spread their Muslim faith and Arab culture. It seems likely that the Russia would also have embraced Islam. If the Arab army had captured Constantinople much of Eastern Europe and possibly Russia could have become part of the Muslim world <ref> Meyendorff, p. 134</ref>. The victory of Leo III prevented the Muslims from entering the Balkans for many centuries and when the Ottomans did, they found that Christianity was too well entrenched, so much so, that almost five centuries of Muslim rule did not lead to the region, becoming Islamized. For this reason, the defeat of the Arab siege was as significant as the Battles of Tours and Marathon in the history of Europe.

==Decline of the Umayyad Dynasty==

It has often been stated that the Umayyad dynasty went into decline after the unsuccessful siege of Constantinople. There is no doubt that the Caliphs had been severely weakened on land and sea. The defeat came at a difficult time for the dynasty as the Empire was also suffering from fiscal challenges. The balance of power after the siege shifted towards the Byzantine. The Umayyad dynasty was so concerned in the wake of the defeat that the considered abandoning their recent conquests in Iberia. The decline of the Caliphs was underlined at the Battle of Akroinon in 740 when once again Leo III defeated a large Arab force when he annihilated some 20,000 soldiers. The blow to Umayyad prestige was significant and many zealous Muslims claimed that the defeats were because of the impiety of the Caliphs. This culminated in a series of revolts that ultimately led to the Abbasid Revolution when the Umayyads were deposed and almost the entire family massacred. The defeat of the Umayyads before the walls of Constantinople helped to undermine the dynasty. However, they Caliphs had also been weakened by a series of defeats in central Asia and in the Caucuses. It cannot be denied that the failure of Umar II to take Constantinople was a factor in the decline and the fall of the second Caliphate <ref>Hawting, G.R. The first dynasty of Islam: the Umayyad caliphate, AD 661–750 (London, Rutledge, 2000), p. 201</ref>.