(g) Among the examples of Advanced Guard work in Marshal Foch's "Principles of War" is a problem for a battalion as the Advanced Guard of a Brigade. "What is the problem the battalion commander has to solve? It consists in preparing for the brigade to go into action against an enemy who may debouch from Bettwiller. What does the brigade require for such an action? It requires the _space_ necessary for the full employment of its forces, and the _time_ necessary for their arrival and deployment. In order to achieve that double task the battalion commander orders his troops to occupy _the whole space necessary_, and places them in points where they may hold on for the _necessary time_."
"A man thoroughly penetrated with the spirit of Napoleon's warfare would hardly fail to make his enemy's communications his first objective."---Col. G. F. R. HENDERSON.
The Flanks are the most vulnerable points of an army, for an attack upon these points subjects the defenders to enfilade fire, and is delivered by troops arrayed in attack formation against an enemy that is not in a position to repel the attack. The consequences of a successful Flank Attack are so far-reaching that every effort will be made by a commander to bring about such a consummation in order that he may sever his adversary's communications, bring him to the end of his resources, and deprive him of the means of replenishing them.
If, therefore, there is any possibility of a column on the march being attacked in flank a force must be detached to protect that flank, and if both flanks are exposed to attack both must be similarly protected. The flank is the most vulnerable part of a moving column, and an attack driven home upon that part has every prospect of success, for it will be delivered by a force that is distributed in depth against a force that is protracted in width after changing front to meet the attack, and the absence of depth in the defending force will deprive the defence of the principal source of strength in resisting attack.
An independent column is liable to attack on either of its flanks, unless the nature of the country through which it is passing provides security for one or the other in the form of an impenetrable feature (such as a wide, { 115} trackless marsh), or an impassable barrier (such as a neutral frontier). The outer columns of a force moving on parallel routes will have an exposed flank, while their inner flank is protected by maintaining touch with the neighbouring column.