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charged by the Byzantine Emperor with the general control of the Arabs of Syria (Masudi: iii., 214-5); that tribe was superseded by the tribe of Salih (id. 216), and that by the Arab kingdom of Ghasan which acknowledged the Emperor of Byzantium as its overlord, whilst the Arab kingdom of Hira acknowledged the Persian king. Somewhere between A.D. 604 and 610, when the first beginnings of persecution were falling on the Prophet in Mecca, the Arabs led by al-Mondir inflicted a crushing defeat upon the Persian army under King Khusraw Parwiz, who, a few years before, had led a victorious force to the invasion of the Byzantine province of Syria. This victory showed the Arabs that, in spite of its imposing appearance, the Persian Empire, and presumably the Byzantine also, were vulnerable, and a determined effort might easily place the wealth of both at the disposal of the Arabs.
The Muslim conquests of the 7th century A.D. form the last of a series of great Semitic outspreads of which the earliest recorded in history resulted in the formation of the empire of Babylon some 2225 years before the Christian era. In all these the motive power lay in the Arabs who represent the parent Semitic stock, the more or less nomadic inhabitants of the barren highlands of Western Asia, who have always tended to prey upon the more cultured and settled dwellers in the river valleys and on the lower slopes of the hills.