When the armies dug in out of sight and began to take elaborate measures to conceal themselves, a successful defense, to say nothing of a successful offensive, depended upon seeing something of what the enemy was doing and where he was. There was only one eye that the army could use which was of any value, and that was the aëroplane. To a man standing on the ground and looking across country there would be absolutely nothing in sight. The trenches themselves would not be visible. All the guns could be easily concealed by screens of trees or leaves. Indeed, at times while a battle was raging, it was not possible in the daytime to see anything except scenery and absolutely not a sound was to be heard. Yet along the hills and valleys there might be half a million men watching each other, waiting for each other to move.

But when an observer rose in an aëroplane and flew over the enemy's lines, a great deal became perfectly clear. The trenches could not be concealed completely. There was always that tremendous weapon, the camera, and a picture faithfully reproduced everything. It could be developed and enlarged, and then, with a microscope and a ruler, the officers could determine the exact location of nearly everything the enemy had prepared. Balloons were also sent up from which an observer could see several miles and from which, of course, the enemy batteries within close range could be detected by the puffs of smoke d hen they fired. The man in the balloon would identify the spot on his map and then telephone the location to his own batteries beneath him.

All this machinery of aëroplanes, balloons, observers, telephones, and wireless was absolutely essential to hit anything, whether for defense or offense. Of course, an aëroplane flying over the enemy lines could see whether the trenches were full of men, and therefore ready for an attack; discover whether or not large numbers of reënforcements were coming up, and therefore give the only warning which was at all dependable of the time and whereabouts of an assault.

The function of the aëroplane was also to put out the enemy's eyes. It must prevent his aëroplanes from cruising over the Allied lines. It must shoot down his balloons; it must kill his observers. Obviously, if either side could get complete control of the air for a time, it might win a great victory, and perhaps the war.

When the army came to consist of millions of men, an ordinary voice became useless for giving orders and any of the older methods by dispatch riders became impossible. Ordinary ears were no longer useful. The war was necessarily fought by wire, by telephone, telegraph, and wireless. From the slightest thing up to the most important order, everything must go over the wires, and the electrical department of the modern army was its most important factor, because it was its voice and its ear. Headquarters looked like a telephone exchange. It would probably be located in some perfectly quiet village quite out of range of any possible gun fire, and even during a great battle no sound would be heard there but the song of the birds and the humming of insects, the low tones of the officers, and the buzzing of the telephone.

No aëroplane was without its wireless, no balloon without its telephone or telegraph, no listening post was created out toward the enemy lines without its wire. And the artillery observer, crawling on his stomach to the top of some hill miles away from his own battery, or perched on top of a tree somewhere, carried with him the invariable wire; otherwise he was lost. But with that wire he might talk to the commanding general himself. The ears of the army spread in all directions and a whisper from the front trenches could reach anywhere.

It was imperative, too, when millions of men went into the field, that the brains of the army should be multiplied. No one man could remember everything; no one man could attempt to make all the decisions; nor would he dare be responsible for everything. An apparently simple order from the commander-in-chief would involve hundreds of detailed orders for its execution, and the execution of that order was the important thing. Foch might decide to attack with a million men on a certain morning. It was the business of the general staff to get a million men there and see that they attacked. The number of orders and details involved was simply incredible. No civilian can possibly imagine what it meant.

The general staff of every army was therefore composed of a great many officers, each of whom had charge of certain parts of the work. Some of them made a specialty of knowing the map; others of watching the location of troops; others had charge of the railroads or of the artillery. Outwardly, the general staff consisted of a little group of gentlemen, living quietly in an inconspicuous house in some perfectly peaceful place miles away from anything that looked like war. The modern general never leads the attack. He is not even within sound of the battle. Indeed, his real work is probably done a week or a month or more before the battle is fought. Hours and days may go by without there being any occasion for him to change an order. He is merely waiting until certain things already decided upon can take place. In the meantime he occupies himself as best he may. He plays tennis or golf or goes fishing. He has to eat his meals and get his sleep even if the battle is going on. He is always in touch with it through the telephone, but he has to see and hear by wire, and he has a general staff whose duty it is to do most of the listening and watching.

The legs of an army of millions cannot be physical. Where the battle line covers hundreds of miles, there is no time to wait while men walk from one part of it to the other. Steam and gasoline were the legs of the army, and in the summer of 1918 the question of victory became almost the question of gasoline. If the supply could be increased, the movement of the army could continue; if the supply gave out, the great offensives which were driving the Germans out of France would come to an end. The gasless Sundays and holidays in America in the summer of 1918 played their part in crushing the Hun. The motor car was one most important leg of the army; the other was the railroad. It was not merely true that everything the army had came by rail or motor car and that it could not be maintained without them; the fact was that the moving of troops in actual fighting from one place to another was made by motor cars and railroad trains. The American Marines charged on motor cars from a point over seventy miles distant from the battle line.

The stomach of the army, as Napoleon pointed out in a famous remark, was its most important organ. "An army fights on its stomach," said he. A hungry soldier cannot fight well and he certainly cannot fight long. If the modern battle was to continue for weeks and months, it was essential that the men should be carefully fed and carefully rested during the battle itself. There must be no interruption in the supply of food and it must be hot food. Think of the amount of food that a million men would eat three times a day and the extent of an organization necessary to deliver that food to each man, wherever he might be, hot and appetizing.

That was one of the greatest tasks that the modern army had to perform It meant the baking each day of tons of bread, the cooking of tons of meat. Whole railroad trains full of supplies were consumed daily. Bakeries covering acres of ground had to be erected. Storehouses, literally miles in length, with acres of space, were built and the task was done and well done, both by the Germans and by the Allies. One of the most remarkable things the American army did was the institution in a few weeks of a great supply department adequate to care for the hundreds of thousands of men whom the United States was rushing to the battlefield in 1918.

The important thing, however, which the French and British had to learn at the beginning of the war, was the need for absolutely exact cooperation between every part of the service. The infantry and the artillery must act together. Every infantry division must cooperate exactly with every other division and with every artillery division. Nothing could go wrong; not a man must fail to perform his task. Not a battery of the artillery must fail. The eyes of the army must see and see accurately. The ears must hear and they must not make mistakes. The voice must repeat over the telephone the exact number and not something else. The brain must be infallible; the legs must never get tired; the stomach must always be full. Otherwise the attack would fail, and because in 1915 all of these various elements of the army did not work together as one organization, some of the attacks did fail.

It would take time, the French and British saw, to perfect the machinery of the modern army and yet nothing short of perfection would suffice. "Muddling through" would not do; "pretty nearly right" was fatal; doing something "to-morrow" probably meant the death of thousands of men. The watches of the generals and the watches of the lieutenants must all read exactly the same and must correspond with the watches of the artillery. If a certain thing was to be done at exactly five o'clock, every watch of every man cooperating in that movement must correspond. For one set of guns to be a minute too early or a minute too late might throw the whole movement into disorder. For an infantry company to leave its trench a minute too soon might result in the death of every man. Literally no detail, however small, could be forgotten.

All these matters were more important and more difficult to achieve than the Allies thought in 1915. They were also more difficult than the Germans thought at this same time. That is one reason why the war lasted four years. So elaborately exact a movement on the part of so many millions of men, in which the failure of one man to do exactly right any one of twenty things might be fatal, took years to learn what to do, years to learn how important it was that it should be done. That part of the history of the war is extraordinarily difficult to appreciate, yet upon it depended the eventual victory.