(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. An example might occur in a situation where the commander has received an order to "Protect the base at A". It is then supposed that, after estimating the situation, he has reached the Decision "to deny the enemy the use of base sites within effective bombing range of A", the purpose of the Decision being, of course, "in order to protect the base at A". The action required might then be undertaken in two stages. The first stage might be confined to the area ABCD. If, then, all available base sites in this area, except Y island, were already securely in friendly hands, the commander would find it necessary to make provision for an operation to deny the use of this island to the enemy. If this operation is of such a nature that the commander desires to execute it under his own direct control, instead of assigning it to a subordinate, it presents a subsidiary problem which the commander, himself, has to solve. The commander has now determined the necessity of solving a subsidiary problem relating to the accomplishment of a designated part of his general plan. He has also determined the necessity of solving another subsidiary problem presented by an operation pertaining to the first stage of the accomplishment of his general plan. Each subsidiary problem requires an estimate of the situation although "the brevity of the mental process tends to obscure this fact" (page 172).