The same considerations point also to the vital importance of due provision, with respect to the armed forces of a State, for joint operations involving concerted action on land, by sea, or in the air. In connection with the capabilities of particular commanders (page 66), it will be appreciated how important it is, more especially in amphibious or joint operations, for responsible officers to have a correct understanding of the powers and limitations of the several types of military forces involved, be their primary medium of movement the land, the sea, or the air. Factors of dispersion and concentration are also involved in apportionment of fighting strength. While undue dispersion may result in lack of adequate fighting strength where required, a certain degree of dispersion may be necessary to meet the demands of movement and of freedom of action. Serious errors in this regard, however, may result in inability to furnish support where needed, and in consequent punishment or isolation of one or more valuable detachments. In distant operations some dispersion is required to safeguard long lines of communication. The requirements for this purpose may sometimes be so great that, unless the total available strength is adequate, a due apportionment to the guarding of long lines of communication may so weaken the main force as to prevent the attainment of the objective. (See also page 63.) Proper dispersion is, therefore, a requirement to be met, while undue dispersion is to be avoided. But realization is also necessary, in this connection, that there is an equal danger in over-concentration. An undue concentration of means at any point may subject such a force to unnecessary loss. Another disadvantage may be lack of adequate fighting strength elsewhere. Accordingly, axiomatic advice that it is unwise to divide a total force, while containing a sound element of caution, is misleading and inadequate, for division is often necessary or desirable. To be adequate, a maxim or rule relating to division of force should indicate when, and in what measure, such division may or may not be necessary or desirable. (See also page 25.) Similarly inadequate, however true as a generality, is the statement that the requirements of effective warfare are met by bringing superiority to bear at the decisive time and place. Such an injunction is of little assistance in solving practical problems as to the appropriate degree of superiority, and as to the proper time and place. In like manner, any rule is faulty which advises a commander to seek the solution of his problems by always bringing to bear his elements of strength against the hostile elements of weakness. It may be found, on occasion, that it is necessary or desirable to act with strength against strength.