And situated, as they were, between two very wealthy empires, the Byzantines and the Sassanians, there was plenty to fight for. The first of all were the Sassanians, the last non-Muslim successor to the Persian empire. They were relatively easy pickings because they'd been fighting the Byzantines for like 300 years and they were super tired. Also they had recently been struck by plague. Plague, man, I'm telling you, it's like the red tortoise shell of history. But in those early days they did pry away some valuable territory like Egypt and the Holy Land and then eventually they got into Spain, where various Muslim dynasties would entrench themselves until being expelled in 1492. But as good as they were at making war, it's still tempting to chalk up the Arab success to, you know, the will of God.