The heavy train, the artillery, the provisioning of the force, can in general only proceed upon good ways or by navigable rivers. At any rate, if the army departs from these, a rival army in possession of such means of progress will have the supreme advantage of mobility. Again, upon the flat an army may proceed by many parallel roads, and thus in a number of comparatively short columns, marching upon one front towards a common rendezvous. But in hilly country it will be confined to certain defiles, sometimes very few, often reduced to _one_ practicable pass. There is no possibility of an advance by many routes in short columns, each in touch with its neighbours; the whole advance resolves itself into one interminable file. Now, in proportion to the length of a column, the units of which must each march one directly behind the other, do the mechanical difficulties of conducting such a column increase. Every accident or shock in the long line is aggravated in proportion to the length of the line. Finally, a force thus drawn out on the march in one exiguous and lengthy trail is in the worst possible disposition for meeting an attack delivered upon it from either side. All this, which is true of the actual march of the army, is equally true of its power to maintain its supply over a line of hills (to take that example of an obstacle); and therefore a line of hills, especially if these hills be confused and steep, and especially if they be provided with but bad roads across them, will dangerously isolate an army whose general base lies upon the further side of them. What the reader has just read explains the peculiar character which the valley of the Upper Danube has always had in the history of Western European war. The Rhine and its tributaries form one great system of communications, diversified, indeed, by many local accidents of hill and marsh and forest, with which, for the purposes of this study, we need not concern ourselves. In a lesser degree, the upper valley of the Danube and its tributaries, though these are largely in the nature of mountain torrents, forms another system of communications, nourishing considerable towns, drawing upon which communications, and relying upon which towns as centres of supply, an army may manoeuvre. But between the system of the Rhine and that of the Danube there runs a long sweep of very broken country, the Black Forest merging into the Swabian Jura, which in a military sense cuts off the one basin from the other. At the opening of the eighteenth century, when that great stretch of hills had but a score of roads, none of them well kept up, when no town of any importance could be found in their valleys, and when no communication, even of a verbal message, could proceed faster than a mounted man, this sweep of hills was a very formidable obstacle indeed.