On September 3, then, the scale of advantage had begun to dip on the side of the defence. It remained to make that advantage decisive. The opportunity speedily offered. Since the opportunity had been looked for, General Joffre had made his dispositions accordingly, and was ready to seize it. Let it be recalled that the most vulnerable and at the same time the most vital point of the German echelon was the outside or right flank of the leading formation, the force led by General von Kluck. Obviously that was the point against which the weight of the French and British attack was primarily directed. To grasp clearly the operations which followed, it is necessary here to outline the natural features of the terrain and its roads and railways. For that purpose it will probably be best to start from the Vosges and take the country westward as far as Paris. On their western side the Vosges are buttressed by a succession of wooded spurs divided by upland valleys, often narrowing into mere clefts called "rupts." These valleys, as we move away from the Vosges, widen out and fall in level until they merge with the upper valley of the Moselle. If we think of this part of the valley of the Moselle as a main street, and these side valleys and "rupts" as _culs-de-sac_ opening off it, we form a fairly accurate notion of the region. From the valley of the upper Moselle the valley of the upper Meuse, roughly parallel to it farther west, is divided by a ridge of wooded country. Though not high, this ridge is continuous. On the points of greatest natural strength commanding the roads and railways running across the ridge, and mostly on the east side of the valley of the Meuse, had been built the defence works of the fortified frontier. Crossing the valley of the Meuse we come into a similar region of hills and woods, but this region is, on the whole, much wilder, the hills higher, and the forests more extensive and dense. The hills here, too, form a nearly continuous ridge, running north-north-west. The highlands east of the Meuse sink, as we go north, into the undulating country of Lorraine, but the ridge on the west side of the Meuse extends a good many miles farther. This ridge, with the Meuse flowing along the east side of it and the river Aire flowing along its west side, is the Argonne. It is divided by two main clefts. Through the more northerly runs the main road from Verdun to Chalons; through the more southerly the main road from St. Mihiel on the Meuse to Bar-le-Duc, on the Marne. Thus from the Vosges to the Aire we have three nearly parallel rivers divided by two hilly ridges. North of Verdun the undulating Lorraine country east of the Meuse again rises into a stretch of upland forest. This is the Woevre.