The application of ordered and classified knowledge of naval warfare starts, naturally, with a consideration of its objectives, and proceeds thereafter to the study of the various classes of operations which may be utilized to this end. Naval effort has as its objective the keeping open of sea communications (see page 62). Command of the sea exists for one belligerent when he possesses and can exercise the ability to move surface traffic, while also being able to prevent the enemy from doing so. Naval warfare, therefore, logically includes operations for the purpose of gaining, maintaining, or disputing command of sea areas, especially under conditions where freedom of movement and the keeping open of sea communications are of vital importance. Such operations may be classified under the headings: (1) For securing command of sea areas, (2) In sea areas not under command, and (3) In sea areas under command. On the basis of this classification, specific operations, broadly considered, appear to be limited in number. As to classification (1), applicable operations are: to destroy the enemy naval forces, to contain them, or to divert them. For (2), applicable operations are: to raid, to make war against enemy trade, to attack or defend naval lines of communication, and to conduct amphibious warfare requiring overseas movement. For (3), applicable operations are: to blockade trade, to defend own coastal and critical areas, to safeguard expeditions against enemy territory, and to carry out offensive operations against enemy coastal objectives. Manifestly, each such operation, broadly viewed, may be considered, in an estimate of the situation, as a course of action. Each such course of action (or operation) will involve, if developed into a more or less complete plan of action, numerous detailed operations which constitute parts of the whole. (See page 37.) There can be no rigid line of demarcation, always applicable, between courses of action and the more detailed operations pertaining thereto. For example, "to raid" may be, in one instance, an operation of such a character, from the viewpoint of the commander, as to be envisaged, correlatively with "to destroy", as one of his courses of action. Yet, in another problem, a raid may be visualized, properly, as a detailed operation pertaining, in a subordinate capacity, to a more comprehensive operation envisaged as a course of action "to destroy". Similarly, what is a broad course of action from the viewpoint of one echelon in the chain of command, may be correctly viewed, on a higher echelon, as a detailed operation. Operations assigned in tasks imposed by higher authority become the basis for the determination of courses of action on the next lower echelon, a procedure which continues throughout the chain of command until specialized, on the lowest echelons, in the form of a simplified routine (see page 84).