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[[File:Siege of con four.jpg|200px|thumb|left| A coin with the portrait of Leo III]]

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. <i>[https://www.amazon.com/gp/product/0913836907/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0913836907&linkCode=as2&tag=dailyh0c-20&linkId=54b9a005dc6c444f0ade105baf1f1862 The Byzantine Legacy in the Orthodox Church]</i> (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 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 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.