Secondly, the high explosives of recent years, coupled with the vast size of this new mobile howitzer shell, is capable, when the howitzer shell strikes modern fortification, of doing grievous damage which, repeated over several days, turns the fort into a mass of ruins.

Thirdly, the difficulty of accurate aiming over such distances and of locating your hits so that they destroy the comparative small area of the fort is got over by the use of aircraft, which fly above the fort, note the hits, and signal the results.

Now, the Germans maintained that under these quite recently modified conditions not even the best handled and heaviest gunned permanent fort could hold out more than a few days.  The French believed that it could, and they trusted in the stopping power not only of individual works (such as the fortress of Manonvilliers on the frontier), but more especially of great rings of forts, such as surround Liege, Namur, Verdun, etc., and enclose an area within the security of which large bodies of troops can be held ready, armies which no one would dare to leave behind them without having first reduced them to surrender.

The very first days of the war proved that the German theory was right and the French wrong.  The French theory, upon which such enormous funds had been expended, had been perfectly right until within quite recent years the conditions had changed.  Port Arthur, for instance, only ten years ago, could hold out for months and months.  In this war no individual fort has held out for more than eleven days.

It might be imagined under such circumstances that the very existence of fortresses was doomed; yet we note that Verdun continues to make a big bulge in the German line four months after the first shots fell on its forts, and that the Germans are actively restoring the great Belgian rings they have captured at Liege, Antwerp, and Namur.

Why is this?  It is because another German theory has proved right in practice.

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