The circle marks the limits within which the capital shifted from the period of the First Dynasty onwards. It was only under the abnormal conditions produced by the Moslem conquest that Kûfa and Basra became for five generations the twin capitals of 'Irâk; this interval presents a parallel to the earlier period before the rise of Babylon.] The secret of Babylon's greatness is further illustrated by still later events in the valley of the Euphrates and the Tigris. The rise of Ctesiphon on the left bank of the river was a further result of the eastward trend of commerce. But it lay immediately opposite Seleucia, and marked no fresh shifting of the centre of gravity. Of little importance under the Seleucid rulers, it became the chief city of the Arsacidæ, and, after the Parthian Empire had been conquered by Ardashir I., it continued to be the principal city of the province and became the winter residence of the Sassanian kings. When in 636 A.D. the Moslem invaders defeated the Persians near the ruins of Babylon and in the following year captured Ctesiphon, they found that city and Seleucia to which they gave the joint name of Al-Madâin, or "the cities," still retaining the importance their site had acquired in the third century B.C. Then follows a period of a hundred and twenty-five years which is peculiarly instructive for comparison with the earlier epochs of Babylonian history. The last of the great Semitic migrations from Arabia had resulted in the conquests of Islam, when, after the death of Mohammed, the Arab armies poured into Western Asia in their efforts to convert the world to their faith. The course of the movement, and its effect upon established civilizations which were overthrown, may be traced in the full light of history; and we find in the valley of the Tigris and Euphrates a resultant economic condition which forms a close parallel to that of the age before the rise of Babylon. The military occupation of Mesopotamia by the Arabs closed for a time the great avenues of transcontinental commerce; and, as a result, the political control of the country ceased to be exercised from the capital of the Sassanian kings and was distributed over more than one area. New towns sprang into being around the permanent camps of the Arab armies. Following on the conquest of Mesopotamia, the city of Basra was built on the Shatt el-'Arab in the extreme south of the country, while in the same year, 638 A.D., Kûfa was founded more to the north-west on the desert side of the Euphrates. A third great town, Wâsit, was added sixty-five years later, and this arose in the centre of the country on both banks of the Tigris, whose waters were then passing along the present bed of the Shatt el-Hai. It is true that Madâin retained a measure of local importance, but during the Omayyad Caliphate Kûfa and Basra were the twin capitals of 'Irâk.[13]