For example: two White army corps, I., II., as in the annexed diagram, each of two divisions, 1, 2, and 3, 4, have to retire before a greatly superior Black force, _abcde_. They succeed in retiring by the action expressed in the following diagram. White corps No. I. first undertakes to hold up the enemy while No. II. makes off. No. I. detaches one division for the work (Division 2), and for a short time it checks the movement of _a_, _b_, and _c_, at least, of the enemy. Now _d_ and _e_ press on. But they cannot press on at any pace they choose, for an army must keep together, and the check to _a_, _b_, and _c_ somewhat retards _d_ and _e_. They advance, say, to the positions δε. [Illustration: Sketch 48.] Next, White corps No. II. stops, puts out one of its divisions (say 4) to check _d_ and _e_, while its other division either helps or falls back, according to the severity of the pressure, and White corps No. I. makes off as fast as it can. _a_, _b_, _c_, no longer checked by a White rearguard, are nevertheless retarded from two causes--first, the delay already inflicted on them; secondly, that they must not, if the army is to keep together, get too far ahead of their colleagues, _d_ and _e_, which White corps II. is holding up. [Illustration: Sketch 49.] Thus, on the second or third day the retreat of White is being secured by an increasing gap between pursued and pursuers. The process is continued. Every succeeding day--if that process is successful--should further widen the gap until White can feel free from immediate pressure. Such is the principle--modified indefinitely in practice by variations of ground and numbers--under which a retirement must be conducted if it is to have any hope of ultimate success in saving the pursued. But it is clear that the process must always be a perilous one. Unless the most careful co-ordination is maintained between the moving parts of the retreat; unless the rearguard in each action falls back only _just upon_ and not a _little while after_ the precise moment when it can last safely do so; unless the new rearguard comes into play in time, etc., etc.--the pursuers may get right in among the pursued and break their cohesion; or they may get round them, cut them off, and compel them to surrender. In either case the retreating force ceases to exist as an army. In proportion as the pursuers are numerous (mobility being equal) compared with the pursued, in that proportion is the peril. And with the best luck in the world some units are sure to be cut off, many guns lost, all stragglers and nearly all wounded abandoned in the course of a pressed retreat, and, above all, there will be the increasing discouragement and bewilderment of the men as the strain, the losses, and the ceaseless giving way before the enemy continue day after day with cumulative effect.