The sudden surging forward of the Arabs was only apparently sudden. For centuries previously the Arab migration had been in preparation. It was the last great Semitic migration connected with the economical decline of Arabia. Such a decline is indisputable, even though we may not be disposed to accept all the conclusions which have in recent times been connected with this oft-discussed thesis. Ever since the commencement of our chronology the Arabs had been in fluctuation. South-Arabian tribes were lords of Medina, others also from South Arabia were settled in Syria and Mesopotamia. Legendary information, confirmed however by inscriptions of Southern Arabia, shews that for a long period the conditions of life in the southern part of the Arabian peninsula had been growing worse. With the decline of political power the care of the public waterworks, on which the prosperity of the land more or less depended, also suffered. In short, long before Mahomet Arabia was in a state of unrest, and a slow, uncontrollable infiltration of Arabian tribes and tribal branches had permeated the adjoining civilised lands in Persian as also in Roman territory, where they had met with the descendants of earlier Semitic immigrants to those parts, the Aramaeans, who were already long acclimatised there.
Persia and Byzantium suffered severely from this constant unrest in their border provinces, and both empires had endeavoured to organise the movement and to use it as a fighting medium, the one against the other. The Romans had organised the Syrian Arabs for this purpose under the leadership of princes of the house of Ghassān, the most celebrated of whom even received the title of patrician, while the Sāssānids founded a similar bulwark in Ḥīra, where the Lakhmites, under Persian sovereignty, lived a princely life, greatly celebrated by Arabian poets. A short-sighted policy, and probably also internal weakness, permitted the ruin of both of these States, which would have offered an almost insuperable barrier to the Islāmitic expansion. The hitherto united dominions of the Ghassānids were subdivided and various governors took the place of the popular Lakhmite princes. Thus the great empires had succeeded in destroying the smaller Arabian States which had grown too powerful, but the tradition remained, according to which the Arabians on the borders might with impunity levy contributions on the neighbouring cultivated countries during the constant wars between Persia and Byzantium. These traditions were assimilated by those Arabs then gradually becoming dependent on Medina, and their procedure was sanctioned and encouraged by the young and rising Caliphate; at first in a wavering, but later in a more and more energetic manner. The expansion of the Saracens is thus the final stage in a process of development extending over centuries. Islām was simply a change in the watchword for which they fought; and thus arose at the same time an organisation which, based on religious and ethnical principles and crowned with unexpected success, was bound to attain an historical
