These considerations, and the fact that the surface of the sea is a broad plane, permit open sea areas to be traversed by a variety of routes to an extent not applicable in the case of land areas and the air above them. In addition, the fact that technological developments have been such as to permit movement, not only on the surface of the sea and through the air above but also beneath the surface, gives distinctive characteristics to the sea when considered as a theater of operations. The surface of the sea has, from the earliest days to the present, provided roads over which human beings in greatest numbers and the resources of the world in greatest weight and volume can be transported in single carriers. From the standpoint of any belligerent it is imperative that, during war, these roads be kept open to the extent demanded by the needs of the State. It is equally imperative that an enemy be deprived of the advantage which their use might otherwise afford. In both cases localized (even though temporary) control, not only of the surface but of the water beneath and the air above, may be essential. It is pertinent, also, to note at this point the interest of neutrals, or of unneutral nonbelligerent Powers, in keeping open the trade routes via the high seas. Such interest may constitute an important factor in the calculations of a belligerent State. Considerations of maximum capacity for speed represent the utmost possibilities with respect to movements (i.e., change of positions) (page 60) in a given medium within a given time limit. A knowledge of maximum speed potentialities, one's own and those of the enemy, is required if changes in position are intelligently to be made. A knowledge of the variety of conditions, controllable and otherwise, which affect or preclude the employment of maximum speed, is likewise a requisite. Poor material condition, inadequate training, and incorrect methods of operation are preventable or correctable. The limitations on speed which are imposed by logistics, and by natural obstacles such as the hydrography, the climate, the wind, the weather, and the state of the sea, are susceptible of greatest possible adjustment to circumstances only by the exercise of foresight and judgment. All these conditions indicate the close relationship that exists between relative position and freedom of action (page 70). The same observations apply to considerations of maximum capacity for endurance, the ability to operate without necessity for replenishment from an outside source. Radius of action is decreased or increased accordingly with resultant restrictions, or otherwise, on freedom of action.