With respect to the freedom of action of armed forces, also a consideration in relation to feasibility, the logistics of a military operation, of whatever scope, constitutes a problem which begins when the plan is in process of formulation. This problem ends only when the necessity for sustaining the movement, and for retaining the position gained, no longer exists. Ships and other means of conveyance, surface, subsurface, and air, are incapable of providing the necessities of life and the implements of warfare beyond the capacity built into them. Operations which extend beyond the limits of such capacity must cease unless replenishment and support, possible only from other sources, are provided. The logistics problem may be so difficult as to cause rejection of a course of action involving distant operations. From the standpoint of supply, military movements by land, sea, and air are, therefore, vitally associated with positions on land and with their relation to the area of operations (see also page 58). The same observations apply in larger scope to the State itself, which, because of economic vulnerability with respect to certain essential raw materials, may be compelled to seek support from outside sources lest supplies on hand become exhausted. In all cases, great importance attaches to the geographical location of sources of supply in their relation to a required point of delivery and to the routes which lie between. It follows that enemy sources of supply may be suitable physical objectives (see page 56). Their destruction or capture, or the severance of the enemy's lines of communication with them, may seriously restrict his freedom of action. From the standpoint of the relative position of its features, and apart from their inherent military value, the characteristics of the theater of military operations may exert an important influence upon the shaping of events. Each characteristic merits consideration as a potential means of facilitating or obstructing movement. Some localities may have been developed as repair, supply, or air bases. Others may be sources of essential raw materials. Certain points may be heavily fortified. Island formations may be valuable to either opponent, or to both, because of the capacity and security of their harbors, the character of their terrain, or their positions relative to each other. The inherent military value of the several features of the theater may be enhanced or vitiated by the relative position which each occupies with respect to other features, and with reference to the location of the armed forces involved. So-called "strategic points", historically significant in connection with military operations, derive their importance by reason of their relative position with reference to routes of movement.