The operative superiority of an enemy is determined by the greater marching capacity of the troops, by the rapid and systematic working of the communications with the rear, and, above all, by the length of the columns of the operating troops. Under the modern system of colossal armaments, an army, especially if in close formation, cannot possibly live on the country; it is driven to trust to daily food-supplies from the rear. Railways are used as far as possible to bring up the supplies; but from the railhead the communication with the troops must be maintained by columns of traction waggons and draught animals, which go to and fro between the troops, the rearward magazines, and the railhead. Since traction waggons are restricted to made roads, the direct communication with the troops must be kept up by columns of draught animals, which can move independently of the roads. The waggons of provisions, therefore, which follow the troops, and are filled daily, must come up with them the same day, or there will be a shortage of food. This is only possible if the troop column does not exceed a certain length and starts at early morning, so that the transport waggons, which, at the end of the march, must be driven from the rear to the head of the column, can reach this before the beginning of the night's rest. The fitness of an army for attack can only be maintained if these supplies are uninterrupted; there must also be a sufficient quantity of tinned rations and provisions which the soldiers can carry with them. If the length of the columns exceeds the limit here laid down, the marches must be proportionately shortened. If unusually lengthy marches are made, so that the provision carts cannot reach the troops, days of rest must be interposed, to regulate the supply. Thus the capacity of an army to march and to carry out operations is directly dependent on the possibility of being fed from the rear. A careful calculation, based on practical experiences, shows that, in order to average 20 to 22 kilometres a day--the minimum distance required from an army--no column on a road ought to exceed a length of about 25 kilometres This consideration determines the depth of the army corps on the march, since in an important campaign and when massing for battle troops seldom march in smaller bodies than a corps.