The conquest of the Persian and half of the Byzantine Empire by the Arabs, under the banner of Islam in the seventh century, was one of the most extraordinary events in the history of the world. On the one side were ranged the forces of two highly-organized military powers, Imperial New Rome and Imperial Persia, which for over three centuries had been engaged in constant conflict with each other. Although this necessarily tended to exhaust the material resources of the combatants, it would naturally be supposed that it must have given them military experience, and their leaders a training in generalship, adequate to enable them to face with confidence of victory enemies hitherto regarded with contempt as mere barbarians. On the other side we see hosts of men, reared in a country where the conditions of life have always been of the hardest and most precarious, divided by tribal feuds and secular hatreds, poorly armed, with no practice in warfare against disciplined foes, and with no allies to swell their legions. Yet from the beginning the progress of the Arabs was one of almost uninterrupted success.