III. This German theory which has proved right in practice is what may be called "the mobile defence of a fortress." It proposes no longer to defend upon expensive permanent works precisely located upon the map, but upon a number of improvised batteries in which heavy guns can move somewhat behind field-works concealed as much as possible, numerous and constructed rapidly under the conditions of the campaign. Such works dotted round the area you desire to defend are quite a different thing to reduce from isolated, restricted, permanent forts. In the first place, the enemy does not know where they are; in the second place, you can make new ones at short notice; in the third place, if a howitzer does spot your heavy gun, you can move it or its neighbours to a new position; in the fourth place, the circumference you are defending is much larger, and the corresponding area that the besiegers have to search with their fire more extended. Thus, in the old forts round Verdun, about a dozen permanent works absolutely fixed and ascertainable upon the map, and covering altogether but a few acres, constituted the defence of the town. Before September was out the heavy guns had been moved to trenches far advanced into the field to the north and east, temporary rails had been laid down to permit their lateral movement--that is, to let them shift from a place where they had perhaps been spotted to a new place, under cover of darkness, and the sectors thus thrown out in front of the old fortifications in this improvised mobile fashion were at least three times as long as the line made by the ring of old forts, while the area that had to be searched was perhaps a hundred times as large. For in the place of the narrowly restricted permanent fort, with, say, ten heavy guns, you had those same ten heavy guns dotted here and there in trenches rapidly established in half a dozen separate, unknown, and concealed spots, along perhaps a mile of wooded hill, and free to operate when moved over perhaps double that front. IV. _In Grand Strategy a German general theory of strategics was opposed to a French general theory of strategics, and upon which of the two should prove right depended, much more than on any of the previous points, the ultimate issue of the campaign._ This is far the most important point for the reader's consideration. It may be said with justice that no one can understand this war who has not grasped the conflict between these two fundamental conceptions of armed bodies in action, and the manner in which (by the narrowest and most fortunate margin!) events in the first phase of the war justified the French as against the German school. I must therefore beg the reader's leave to go somewhat thoroughly into the matter, for it is the foundation of all that will follow when we come to the narration of events and the story of the Western battle which began in the retreat from the Sambre and ended in the Battle of the Marne.