In defence, the measures required are comparatively simple. The conformation of the ground usually chosen with a view to the peculiarities of defensive action will often allow of the led horses being kept close to the firing-line, where it will also be possible to mount out of the enemy's range and to withdraw from his fire under cover of the ground. The led horses should only be left to the rear if the position is to be maintained until the enemy comes to close quarters, and then should be, if possible, behind some fresh position, where it seems likely that resistance may be renewed. In defence, the led horses will generally be left quite immobile,[23] though this method should not be considered imperative. For even in defence the led horses must be sometimes kept mobile; for instance, when it is necessary to rapidly occupy on foot some locality in the neighbourhood of which there is no suitable position for the led horses. In such a case they will have to be sent behind cover, which can only be done if they are mobile. The horses will only be kept in the immediate neighbourhood when it is intended merely to bring off a fire surprise, and not when carrying out an attack or making an obstinate resistance (471-473).[24] For the attack the conditions are different. The troops must then be often completely separated from their horses. It is therefore all the more necessary that the latter should be secured against all hostile attack. If, for instance, hostile cavalry threatens, they must have a special escort. The possibility of a reverse must be taken into consideration when selecting the position for led horses, and the retiring troops must be able not only to mount, but to withdraw under cover. It is of especial importance that the troops should regain their mobility after an attack has been carried out. When the led horses can be moved, this is an easy matter. Where, however, they are immobile, as will generally be the case where full force has to be put into the fight, there will be considerable loss of time unless it be possible for the led horses to be brought up, part at a time, by any mounted reserve that may have been left in the rear. In spite of this disadvantage, the desire to render the led horses mobile should not be allowed to lead to the undertaking of a dismounted action with insufficient numbers.