But the real strength of her position lay in her near neighbourhood to the transcontinental routes of traffic. When approaching Baghdad from the north the Mesopotamian plain contracts to a width of some thirty-five miles, and, although it has already begun to expand again in the latitude of Babylon, that city was well within touch of both rivers. She consequently lay at the meeting-point of two great avenues of commerce. The Euphrates route linked Babylonia with Northern Syria and the Mediterranean, and was her natural line of contact with Egypt; it also connected her with Cappadocia, by way of the Cilician Gates through the Taurus, along the track of the later Royal Road.[1] Farther north the trunk-route through Anatolia from the west, reinforced by tributary routes from the Black Sea, turns at Sivas on the Upper Halys, and after crossing the Euphrates in the mountains, first strikes the Tigris at Diarbekr; then leaving that river for the easier plain, it rejoins the stream in the neighbourhood of Nineveh and so advances southward to Susa or to Babylon. A third great route that Babylon controlled was that to the east through the Gates of Zagros, the easiest point of penetration to the Iranian plateau and the natural outlet of commerce from Northern Elam.[2] Babylon thus lay across the stream of the nations' traffic, and in the direct path of any invader advancing upon the southern plains. That she owed her importance to her strategic position, and not to any particular virtue on the part of her inhabitants, will be apparent from the later history of the country. It has indeed been pointed out that the geographical conditions render necessary the existence of a great urban centre near the confluence of the Mesopotamian rivers.[3] And this fact is amply attested by the relative positions of the capital cities, which succeeded one another in that region after the supremacy had passed from Babylon. Seleucia, Ctesiphon and Baghdad are all clustered in the narrow neck of the Mesopotamian plain, and for only one short period, when normal conditions were suspended, has the centre of government been transferred to any southern city.[4] The sole change has consisted in the permanent selection of the Tigris for the site of each new capital, with a decided tendency to remove it to the left or eastern bank.[5] That the Euphrates should have given place in this way to her sister river was natural enough in view of the latter's deeper channel and better water way, which gained in significance as soon as the possibility of maritime communication was contemplated.