You must be able, as you retreat, to check your enemy appreciably before you can trace such a line; you must be able to hammer him badly with heavy guns stronger than his own while you are making it, and unless you are present in very great numbers you will only be able to draw it over a comparatively short line which your enemy may be able to turn by the left or the right. Still, it may be of interest to compare the length of lines thus drawn apparently during the course of a campaign in the past with those drawn in the course of the present campaign, and in the first diagram I show the contrast. It is striking enough. Another novel feature in which this war differs even from the Balkan War is the new value which has been given to howitzer fire, and in particular to its domination over permanent fortification. This is perhaps the most important of all the changes which this war has introduced into military art and it is worth while understanding it clearly. Its main principles are simple enough. Mankind at war has always used devices whereby he has been able with a small number to detain the advance of a larger number. That, for instance, was the object of a castle in the Middle Ages. You built a stronghold of stone which the engines of that time could not batter down or undermine save at a very great expense of time, and you were certain that for every man able to shoot an arrow from behind such defences ten men or more would be needed for the work of trying to batter them down. So when you knew that your enemy would have to go through a narrow pass in the mountains, let us say, or across an important ford of a river, you built a castle which, as the military phrase goes, “commanded” that passage; that is, you devised a stronghold such that with, say, only 1000 of your men you would quite certainly hold up 10,000 of your enemy. If your enemy passed by without taking your castle the thousand men inside could sally out and cut off his supplies as they passed down the mountain road or across the ford, and so imperil his main forces that had gone forward. Your stronghold would never, of course, suffice to win a war--its function was purely negative. You could not attack with it; you could not destroy your enemy with it. But you could _gain time_ with it. You could check your enemy in his advance while you were gathering further men to meet him, and sometimes you could even wear him out in the task of trying to reduce the stronghold. Now the whole history of the art of war is a history of the alternate strength and weaknesses of these _permanent fortifications_; the word _permanent_ means fortifications not of a temporary character, hurriedly set up in the field, but solidly constructed over a long space of time, and destined to permit a prolonged resistance.