The start of the decline of the Eastern Roman Empire began on August the 20th, 636, when a "Roman" force of 40,000 was defeated by an Arab force of 20,000 after 12,000 "Roman" Arab Christians switched sides. The battle was close, and it was decisive; it marked the beginning of the relentless Arab expansion of the 7th and 8th centuries. I wonder, what would have happened had the Arabs been trounced and bloodied at that battle? I was hoping other would be willing to shed light on their perception of the issue.