In this problem, the situation summarized is an imaginary one. It may eventuate either through the natural future developments of the situation existing at the time of the solution of the basic problem, or it may confront the commander during the execution of the plans derived from the Decision of that (basic) problem. The Battle Plan finally to be formulated will be for use under the conditions assumed in this situation.
The commander will desire to draw up a Battle Plan as a provision for the situation which he believes most likely to eventuate. However, as he cannot be certain that this situation will occur, he may also desire to assume other situations, i.e., prepare in advance for other contingencies. It is then necessary for him to solve several problems, each differing from the others in the assumptions ([page 155]) as to the form the situation may take. The summary of the situation therefore requires a brief statement of the conditions which are assumed. In addition, such parts of the basic problem may be included as are deemed pertinent to the new problem in hand.
In his new problem the purpose of the (subsidiary) mission may readily be obtained from the basic problem. Suppose the assigned task, motivating the estimate of the basic problem, to have been to "prevent enemy convoy from reaching destination". This, the motivating task of the basic problem, then becomes a suitable (subsidiary) purpose for the mission of the subsidiary problem.
For the mission of the subsidiary problem, a motivating task, suitable to the purpose thus determined, will be found in the Decision of the basic problem. Suppose the Decision in this case to have been "to destroy the enemy convoy". The task thus determined for the subsidiary problem becomes an assigned task in the sense that it is assigned by the commander to himself, instead of to a subordinate; however, it is also an assigned task in the sense that it has been indirectly assigned by the immediate superior, because it has been derived, in the basic estimate, from the motivating task which was directly assigned by the superior.
The two elements, of task and purpose, when linked together, enable the commander to visualize the appropriate effect desired, as the basis for his subsidiary estimate,—a procedure identical with that followed in a basic estimate. As in the latter, the commander can now formulate his subsidiary mission, as:—
(Task) To destroy the enemy convoy,
(Purpose) in order to prevent it from reaching its destination.
The mission of the subsidiary problem is thus seen to be identical with the basic Decision linked to the purpose of that Decision.
However, this is not always the case. A subsidiary problem may merely involve the execution by the commander, i.e., under his own immediate direction, of a designated part of his general plan. Or, such a problem may involve execution, by the commander, of one or more of the detailed operations for the accomplishment of his general plan or of a part thereof. The commander may also find it necessary to solve numerous subsidiary problems of relatively restricted scope pertaining either to his general plan or to a part thereof or to the detailed operations involved.
In some of these cases the purpose of the subsidiary mission may be readily apparent. In others, its nature may become clear only after the application of considerable mental effort. In every case the determination of a proper (subsidiary) purpose involves visualization of a situation which the commander desires to bring about or to maintain. The (subsidiary) task, appropriate to the (subsidiary) purpose, will always necessarily be suitable to the latter. This task is then the motivating task for the solution of the particular subsidiary problem in hand. This will be the case whether the commander makes a simple mental solution or produces a more complex one in which the formal written estimate of the situation is employed. In the former instance, the brevity of the mental process tends to obscure this fact.