The rise of the caliphate at the expense of the Romans (and Persians) seems a lot less shocking after listening to the History of Byzantium. The Romans thought

The rise of the caliphate at the expense of the Romans (and Persians) seems a lot less shocking after listening to the History of Byzantium. The Romans thought the deserts were a natural southern border. But they became a giant highway in the face of unifying desert tribes. It's quite similar to how the Vikings inverted the strategic safety of the sea and rivers in the north.