Mecca, the city where Muḥammad was born, lies in the middle of the Arabian Peninsula, an immense expanse of land abutting the territories of the most ancient civilizations. Larger than India or Europe, the Arabian Peninsula is almost 1.5 million square miles. Mostly a desert, apart from the southwestern highlands of Yemen, and devoid of any rivers or easily accessible fresh water, Arabia was impenetrable and impossible for armies to march through. It was also an inhospitable and unprofitable land; with no agricultural base to tax, Arabia was spared the avarice of imperial authorities until the struggle between Byzantium and Sassanid Iran became global and drew Arabia into its orbit. Arabia was thus peculiarly unique: in proximity to the most centralized empires ever to exist in the ancient world, yet never under the direct control of any. By Muḥammad's time, the Arabian Peninsula was nevertheless experiencing the pressure of the two major empires on its sides, the Byzantine Empire to the north, comfortably Christian if riven by Christological controversies, and the religiously diverse Sassanid Empire to the east, reviving its ambition to take control of the eastern parts of the Mediterranean from Byzantium.