How did Arab armies defeat the Sassanids and Byzantines?

Large segments of the Eastern Empire and the Sassanian Empire changed hands and both empires lost many of their best levies, thematic troops, and nobles. Along came the vigorous Arabs who were strong and ascending. They smashed the Sassanid armies in three major battles. read more

Whilst the Byzantines and the Persians had more people, more wealth and more centralised forms of government, during the Arab Conquests they were not able to use their resources effectively. Morale was low and the leadership was exhausted, if it still existed. read more

The Sassanids had already been embroiled in a costly and attritional war against Byzantines. Both these empires wages an exhaustive and mutually ruinous war. This was the Byzantine-Sassanid war [602 AD - 628 AD]. read more