On the map are shown the four practically parallel lines of trench defenses which the Germans had constructed between the French border and the battle line. The two anchors of this long line, which stretched from the mountains to the sea something over six hundred miles long, were Metz, where all four of the lines ended, and Lille to the north, where three of them terminated. The loss of Metz would break all four lines, but the Germans regarded that as unthinkable. The loss of Lille would probably destroy the first and second lines, but not the third and fourth, which were devised expressly to meet the emergency of its loss. The whole system was also constructed so as to provide against disaster by an Allied success in any particular sector.

The crosses on the map connecting these various trench lines represent what were called switch lines. They were intended to divide the territory into compartments or rooms like the water-tight compartments in a ship. The loss of any one, or perhaps two, of them would not sink the ship. The others would keep it afloat. These switch lines, therefore, were to limit the success of the Allies; when they should carry some portion of the first trench lines, they would still find the Germans prepared to fight them and hold the remaining sections of the first lines.

The first line, called the Hindenburg Line, was strongest of all and was really a zone twelve miles broad at most places and comprised of many trench lines, one behind the other, connected by switch lines of various sorts on just the same principles as the larger system. Elaborate barbed wire entanglements had been created in front of all the trenches. Deep pits had been dug, covered by planks and sod so as to look like solid ground, into which it was hoped tanks would fall. Mines and bombs had been planted which could be exploded as the Allied troops passed. The trenches themselves were of concrete and were believed to be too heavy for the Allied artillery to destroy by any length of "preparation."

Underground at various points were complete houses for hundreds of troops, with dormitories fitted with beds, kitchens, dining rooms, rest rooms, the whole lighted by electricity. In some of these underground houses at least the electric power was provided in characteristic German manner, by a treadmill worked by man power! They compelled prisoners to tramp round on the treadles for hours at a time so that the officers might have light and heat!

The officers' quarters along the Hindenburg Line were safe from shell fire, and so well constructed that they did survive the final attack. When the Allied troops entered them, they found them furnished with brass beds, comfortable chairs and tables, with pictures on the walls, rugs on the floor, and all the appliances of an ordinary house. The Germans meant to live on the Hindenburg Line, if necessary, for years, and they proposed to be comfortable. They proposed also to be safe, and therefore to live underground. The Allied artillery might pound the surface as long as it liked, for as many days and weeks as it chose, but it would not disturb the real defenses in the least. This being true, the Germans did not see how the Hindenburg Line could be carried. They had also used the rivers, canals, and marshes of the district with great cleverness to furnish sections of the lines with water barriers which prevented the use of tanks, for the tank is a land battleship and does not swim.

The problem of Foch was great because the Allies had attempted many times in the past years to carry sections of the Hindenburg Line, and, while at an enormous cost of life and effort some small gains had been made, no section of the line itself had been carried. The secondary defenses had always held, and, being twelve miles wide, only an extraordinary drive could pass clear through the zone.

What gave Foch courage and confidence was the new technique of warfare which the Allies had developed. Trench warfare had been at first so new that neither the Germans nor the Allies had understood it. And the history of the war had been during its first years one of experiment with the technique of the new weapons. It had taken the generals time to find out what could not be done; it had taken the troops time to learn how to execute movements which were possible. The German campaign of 1918 made it clear that they had solved the question of breaking through the trenches and fighting through into the open beyond. Foch believed that the Allies had also solved it, and that the Hindenburg Line itself would offer no real obstacle.

The greatest difficulty to meet had been the machine guns concealed in concrete shelters. Human flesh could not stand before machine gun fire, and the artillery, however accurate its aim, had proved itself unable to destroy the kind of shelters that the Germans had constructed. However perfect, therefore, had become the technique of the Allies in other ways, the concrete shelter with its machine guns always prevented the continuation of the advance. It was with this sort of arrangement that the tanks were intended to cope. They were to roll in upon these concrete shelters and clean them out. The first tanks, however, were large, slow, and clumsy. The German aëroplanes were quick, and presently the German gunners began to drop high explosives on the tanks, and the moment the technique of the German large artillery became capable of destroying the big tanks, their usefulness could be only partial. Nor could they assault positions protected by water nor yet operate in mud or in wet weather.

The French therefore invented a new kind of tank, called the whippet, a small tank holding only two or three men, moving with great rapidity, and so much lighter than the others that it could move over rougher and wetter ground. It was nevertheless sufficiently well armored to withstand machine gun bullets and swift enough to be far out of the way before large shells came over. Here was a method of dealing with this final obstacle.

The new Allied attack therefore need not start with a long artillery bombardment of days or hours, which merely advertised what was coming and when. A brief but intense fire could now accomplish all the destruction possible aboveground at any moment. Poison gas could then be started, if the wind was right, and both would keep the Germans underground. What was called a creeping or rolling barrage would then be laid down by the artillery. The Allied guns would fire all together and drop on a certain line on the German trenches a row of shells that would create a zone of solid fire. This would prevent the Germans from coming out of their dugouts and incidentally destroy the barbed wire entanglements in front of the troops.

Behind the barrage would come the Allied infantry; slowly the barrage would move forward, the Allied gunners lengthening their range; with the troops would be the whippet tanks, which would dash forward and clean out the machine guns as they revealed themselves. Thus line after line of the trenches could be carried. Of course, the utmost accuracy and foresight were necessary for any such movement. The aëroplane work must be perfect, must tell where the Germans were, and show the artillery exactly what to do. The artillery must hit exactly what it aimed at. The troops must move in exactly such a way to exactly such spots. With these methods, Foch felt sure the whole German defense system could be destroyed—and he was right.

At a time when things looked black in the summer of 1918, Foch was asked by a distinguished man whether he really had a plan for winning the war. General Maurice, the British official observer, tells of the great general's reply. Laconic always and sparing of words, he employed only gestures. He struck out with his left arm, then with his right, then more rapidly with the left again, and finally gave a tremendous kick with his right leg. Three rapid blows and a kick—such was indeed his strategy.