According to the notions that prevailed up to that time, the possibility might have been considered, where our troops were suffering heavy losses as a result of holding on under exposure to the fire of the enemy's heaviest artillery and bomb-throwers, and where the latter had done destruction to our trenches, of allowing the enemy to break through, and then driving him back again by means of the reserves at the back of our lines. This procedure was, in fact, from the beginning employed several times with success at various sections of the front against bodies of the enemy forces which had broken through. To extend it systematically to larger sections of the front, and thereby on our side to resort to a certain extent to the methods of the war of movement, seemed to the Supreme Command for a long time inadvisable, in view of the limited forces and artillery at their disposition. Experience had, moreover, shown how difficult it is to straighten out salients which have once been formed on an entrenched front. Even when salients have been enveloped, they have, by the very nature of modern methods of fighting and effectiveness of weapons in entrenched warfare, been held both by ourselves and by our enemies, in so far as the nature of the ground made this possible. The more and more insistent attempts of our enemies to prepare the way for their infantry by the mechanical power of bomb-throwers and heavy artillery led to a different method of defensive fighting. The German Army Report of the 17th of April, 1917, describes it briefly in the following words: "In the presence of modern artillery fire, which flattens out positions and produces broad deep craters, rigid defence is no longer possible. The struggle is no longer for a line, but for a whole deeply echeloned fortified zone. So the contest for the foremost positions surges this way and that, with the aim, even if it involves the loss of implements of war, of saving the lives of the men, and at the same time of weakening the enemy by inflicting on him severe and sanguinary losses." This procedure preserved the lives and at the same time the morale of the troops, who now no longer saw themselves to the same extent as hitherto exposed without means of defence to the devastating fire of the enemy. The enemy could be allowed to boast of his slight local successes, if only his attempts to break through were frustrated. It remained none the less a prerequisite condition of this new procedure that adequate reserves of troops for the counter-thrust, as well as munitions, should be at hand. Deficiencies in both these respects were revealed more than once in the defensive engagements of the years 1915 and 1916 on the Western front.