The forces of the Rashidun Caliphate achieved a decisive victory against the Byzantines at Yarmouk in 636. This allowed the Muslims to take over Syria and Egypt. But the Eastern Roman Empire was still strong and continued its resistance. The next 80 years Byzantines fought against the onslaught and the Arab-Byzantine wars reached their peak during the Siege of Constantinople in 717-718 where emperor Leo assisted by the Bulgars of Khan Tervel faced the overwhelming odds against the Umayyad forces. This battle is often overlooked in comparison to the battle of Tours that happened in France, but it was bigger and scale and had an even bigger impact on the fate of Europe.
