The Islamic Conquests

In Empire to Commonwealth (12-19), historian Garth Fowden has delineated three separate geographical, cultural, and political spheres in Eurasia: China, India, and the Near East (the Fertile Crescent and the adjacent Mediterranean coast). These areas were all ancient centers of civilization and because of the distances and geographical barriers involved, had little interaction with each other. This is why China, India, and the Near East/Mediterranean can be thought of as continuous discrete civilizations even though throughout much of their history they lacked the political unity to provide full national identity.
Although Islam eventually spread to all three areas, it origin and center lay in the Near East and it was only there that it became fully dominant. The Near East is unique among the three in having close interaction with a wide variety of surrounding areas. The Fertile Crescent is "a vortex that pulls inward and fuses what lies around it. So not only can the Fertile Crescent never enjoy long-term autonomy, but its unity can only be realized on a secure bases as part of a wider unification of the Iranian Plateau with the Mediterranean" (Fowden, 18). This unity was only achieved twice, in the first instance by the Achaemenid Empire (Alexander’s fleeting unification of an even larger territory was based on Achaemenid Iran) in the sixth to fourth centuries BC, and in the seventh century by the Umayyad Caliphate. The initial Arab conquests unified the entire Fertile Crescent region with Iran for the first time in centuries, providing a secure economic, political, and geographical base for further conquests, for instance by allowing the Islamic state to compete on an even footing with the Byzantine navy (Fowden, 140). It may well be that the impetus to sustain large scale conquests into the eighth century came from the Islamic practice of providing for the army from the income of conquered lands collectively, rather than distributing the lands. This tended to keep the army in being (Karsh, 24).
How did the initial Arab conquest succeed so well The Arabs’ opponents in the area, the Byzantine and Sassanian Empires, were exhausted after a generation of fruitless war with each other over the Fertile Crescent and were in internal states of near collapse. In Iran, ultimately the loosing party in this conflict, on which the pressure was increased by Turkish invasions from the north (Christian, 260-285), the aftermath of defeat had led to assassination, civil war, and by 633 a state of anarchy in the empire (Nafziger and Walton, 18). Rome was also riven with internal dissensions (see below). So, to a large degree from a military perspective, the rapid Arab conquest of the Iranian state and of nearly half the territory of the Byzantine state, is to be attributed to the internal weakness of the defeated Empires rather to any special qualities of Islam.
Other possible purely military factors to explain the Islamic conquests have been proposed, though with less plausibility. In Islam at War, Nafziger and Walton suggest a ‘great man’ solution to the problem of the Arab conquest, arguing that Khalid ibn al Waleed was "one of the great [sic] natural military leaders in all of human history" (16-17). But this hardly seems plausible. However remarkable Khalid’s successes were on an operation level, and as astounding as his victories were, the were (as above) due more to the