All Formations should have such a strength and composition as to be in the best relation and proportion to each other, and to the larger groups which they help to build up. Every Formation should be formed of at least three subordinate Units. This gives the Commander of the whole due importance over his Subordinate Commanders, and ensures his retaining an adequate Command whenever he wishes to detach one of his Units. This would not be the case were there only two Units in the whole, for, if one were detached, the Commander of the whole would be left exercising Command only over the other Unit, already adequately commanded. The Superior Commander would then be superfluous, and harmfully interfering with his subordinate. A Formation with three or more Units can be readily broken up when desired, without affecting the principles of Command, and is therefore more flexible and efficient than one with only two Units. Emphasis is laid on this point by Clausewitz in his classic work “On War.” It is the purpose of the next few chapters to describe the Units and Formations constituted in modern armies. But, in order to explain the reasons which have dictated their strength and composition, it is necessary first to describe the various kinds of Troops which go to make up an Army, and their respective methods of fighting, and functions in war. Organization exists to facilitate fighting, and cannot be explained without some discussion of Tactics. CHAPTER II THE FIGHTING TROOPS Military Forces are of two distinct categories: _Fighting Troops_, which carry out the actual operations; _Administrative Services_, whose function is to provide the Fighting Troops with all that they require to keep up their strength and efficiency. THE ARMS OF THE SERVICE The Fighting Troops consist mainly, as they have for centuries, of what are known as “The Three Arms of the Service”--Cavalry, Artillery, and Infantry. Besides these, however, the introduction of warlike inventions and the increased complexity of modern war have brought into being a fourth Arm--Engineers--as well as varieties of fighting troops for special purposes, which are virtually new Arms, such as Mountain Artillery, Machine Guns, Cyclists, and Mounted Infantry. CHARACTERISTICS OF THE ARMS The continued existence of the Arms of the Service for centuries is due to a gradual differentiation of their mode of fighting, owing to changes in weapons, and progress in the Art of War. Each Arm has its peculiar fighting characteristics and its own sphere of action in war, which will be discussed in this chapter. In the next will be described the organization which each Arm has evolved in order to enable it to carry out its functions in war. 1. CAVALRY ITS SPECIAL FUNCTIONS