similar footing. Although we may not be disposed to accept the traditional account given by Bukhari, there no doubt was some such challenge. But Heraclius had only recently re-conquered Syria for the Byzantine Empire, the land he had acquired included a considerable portion of the Syrian desert which formed a geographical unity with Arabia, and amongst his subjects were Arab tribes closely akin to those of the Hijaz.
Islam became a militant religion because it spread amongst the Arabs at a time when they were beginning to enter upon a career of expansion and conquest, and this career had already commenced before Muhammad had got beyond the first—the purely spiritual—stage of his work. The only reason why the earlier Arab efforts were not followed up immediately seems to have been that the Arabs were so surprised at their success that they were unprepared to take advantage of it. For some time previously Arab settlements had been formed in the debateable land where the Persian and Byzantine Empires met, but this encroachment had been more or less veiled by the nominal suzerainty of one or other of the great states. The Quda, a tribe of Himyaritic Arabs, had settled in Syria and become Christian, and was charged by the Byzantine Emperor with the general control of the Arabs of Syria (Masudi: iii., 214–5); that tribe was superseded by the tribe of Salih (id. 216), and that by the Arab kingdom of Ghasan which acknowledged the Emperor of Byzantium as
