It was otherwise with the plan of campaign itself, and there are details that will remain in question till all the archives are opened. Yet this now appears the only plan on which Germany could hope to bring an aggressive war to a successful issue. A repetition of the triumph of 1870 would not be enough, for, if France resisted as long this time, everything would be put in doubt. The blow must be still more swift and overwhelming. To be overwhelming, it must at once reach not portions, but the chief mass, of the French armies. But nowhere in the world had military art, working upon a favourable terrain, set up so formidable a series of obstacles to grand-scale manœuvre as along the line of the Meuse and Moselle Heights and the Vosges. A piercing of this line at the centre, between the fortified systems of Verdun–Toul on the north and Epinal–Belfort on the south, might be an important contributory operation; in itself it could not give a speedy decision. A mere diversion by Belgium, in aid of a main attack in Lorraine, would not materially alter this calculation. The full effects of surprise, most important of all factors in a short struggle, could only be expected where the adversary was least prepared, which was certainly across the north. These offensive considerations would be confirmed by a defensive consideration: German Lorraine, also, was so fortified and garrisoned as to be beyond serious fear of invasion. In neither direction could Alsace provide favourable conditions for a great offensive.
The political objects of the war being granted, these arguments would lead to the strategical conclusion: the strongest possible force will be so deployed, on a vast arc stretching from southern Lorraine to Flanders, that its superiority may at once be brought fully into play. The method was a variant drawn from the teaching of Clausewitz and Schlieffen. The “march on Paris” occupied in the plan no such place as it long held in the popular imagination. The analogy of closing pincers has been used to describe the simultaneous onset of seven German armies ranged in a crescent from the Vosges to Brussels; but it is uncertain whether the southern wing was originally intended to participate immediately in the destructive stroke, or whether this purpose followed upon the collapse of the first French offensives. The latter supposition is the more probable; and we may, therefore, rather picture a titanic bolas ending in five loaded cords, of which the two outer ones are the most heavily weighted. These two outer masses were (_a_) Kluck’s and Bülow’s Armies on the west; (_b_) the Crown Prince of Bavaria’s and Heeringen’s on the east. Approximately equal, they had very different functions, the road of the one being open, of the other closed; the one, therefore, being essentially offensive, the other provisionally defensive. Between these two masses, there were three lesser forces under Hausen, the Duke of Würtemberg, and the Imperial Crown Prince. While the eastern armies held the French forces as originally concentrated, the western mass, by an immense envelopment, was to converge, and the three inner bodies were to strike direct, toward the north-centre of France——perhaps toward the upper Seine, but there could hardly be a precise objective till the invasion developed[2]——destroying any resistance in their path. The eastern thrust which actually followed appears, on this hypothesis, as an auxiliary operation rather than part of a double envelopment: we shall see that, delivered at the moment when the Allies in the west were being driven in between Le Cateau and Givet, it failed against a successful defence of the only open road of the eastern frontier, the Gap of Charmes, and that it again failed a fortnight later. The other German armies went triumphantly forward. In every part of the field is evident the intention to conceal, even to hold back, the movements of approach, and so to articulate and synchronise them that, when the hour of the decisive general action had arrived, there should be delivered a single, sudden, knock-out blow.