But that was not the only cause of Babylon's deposition. For after her capture by Cyrus, new forces came into play which favoured a transference of the capital eastward. During the earlier periods of her history Babylon's chief rival and most persistent enemy had lain upon her eastern frontier. To the early Sumerian rulers of city-states Elam had been "the mountain that strikes terror,"[12] and during subsequent periods the cities of Sumer and Akkad could never be sure of immunity from invasion in that quarter. We shall see that in Elam the Western Semites of Babylon found the chief obstacle to the southward extension of their authority, and that in later periods any symptom of internal weakness or dissension was the signal for renewed attack. It is true that the Assyrian danger drew these ancient foes together for a time, but even the sack of Susa by Ashur-bani-pal did not put an end to their commercial rivalry. During all this period there was small temptation to transfer the capital to any point within easier striking distance of so powerful a neighbour; and with the principal passes for eastward traffic under foreign control, it was natural that the Euphrates route to Northern Mesopotamia and the Mediterranean coast should continue to be the chief outlet for Babylonian commerce. But on the incorporation of the country within the Persian empire all danger of interference with her eastern trade was removed; and it is a testimony to the part Babylon had already played in history that she continued to be the capital city of Asia for more than two centuries. Cyrus, like Alexander, entered the city as a conqueror, but each was welcomed by the people and their priests as the restorer of ancient rights and privileges. Policy would thus have been against any attempt to introduce radical innovations. The prestige the city enjoyed and the grandeur of its temples and palaces doubtless also weighed with the Achæmenian kings in their choice of Babylon for their official residence, except during the summer months. Then they withdrew to the cooler climate of Persepolis or Ecbatana, and during the early spring, too, they might transfer the court to Susa; but they continued to recognize Babylon as their true capital. In fact, the city only lost its importance when the centre of government was removed to Seleucia in its own immediate neighbourhood. Then, at first possibly under compulsion, and afterwards of their own freewill, the commercial classes followed their rulers to the west bank of the Tigris; and Babylon suffered in proportion. In the swift rise of Seleucia in response to official orders, we may see clear proof that the older city's influence had been founded upon natural conditions, which were shared in an equal, and now in even a greater degree, by the site of the new capital. [Illustration: FIG. 1. DIAGRAM TO ILLUSTRATE THE POLITICAL CENTRE OF GRAVITY IN BABYLONIA.