Long before the Christian era the Arabs, a people of Semitic origin, occupied the Arabian peninsula and the Syrian desert which lies to the north of it and stretches as far as the Euphrates River. The peninsula of Arabia, embracing an area equal to approximately one-fourth of Europe, is surrounded by the Persian Gulf on the east, the Indian Ocean on the south, and the Red Sea on the west; in the north it runs gradually into the Syrian desert. Historically, the best-known provinces of the peninsula were (1) Nedjd, on the central plateau; (2) Yemen, or Fortunate Arabia, in the southwest of the peninsula; and (3) Hidjaz, the narrow strip along the coast of the Red Sea, extending from the north of the peninsula to Yemen. The arid land was not everywhere habitable, and the Arabs, who were a nomadic people, occupied chiefly central and northern Arabia. The Bedouins, who were nomads, considered themselves the pure and genuine representatives of the Arabian race and the true bearers of personal dignity and valor. They treated with arrogance and even with contempt the settled inhabitants of the few cities and hamlets.
The Roman Empire was inevitably bound to come into collision with the Arabian tribes on its eastern Syrian border, which it was forced to protect. For this purpose the Roman emperors erected a line of border fortifications, so-called Syrian limes which resembled, on a small scale, of course, the famous limes romanus on the Danubian border, erected for defense against Germanic attacks. Some ruins of the principal Roman fortifications along the Syrian border survive at present.