The success of an action is often promoted by sending out a detach ment with directions to fall on the flanks or rear of the enemy during the engagement. The sudden appearance of a body of troops in such a situation cannot fail to produce embarrassment in the army which is attacked, and to diminish the energy of its operatious towards the front. On the other hand, there is some danger in sending out Large detachments from an army, as it is seldom possible to afford them due support; and therefore they may be cut off by the enemy. The dis tance which the detachment has to march, together with the state of the roads on which it must move, should be ascertained with precision, in order that it may be at the appointed post at a seasonable moment ; and such determinations are very uncertain, particularly if the corps has to make a great circuit. It almost always happens that the de tachment arrives too late for the accomplishment of the object ; and this was the case with a detachment sent by the king of Prussia during the action at Torgau (1760), with a view of turning the left of the Austrians, end cutting off their retreat.
Detachments are however constantly sent out to protect the parties reconnoitring a country, to guard s convoy, or to support a foraging party. In these cases its object is less to fight than to cover a retreat ; therefore the troops advance with circumspection, and retire when the enemy appears in superior force. During the war in Spain (1813), Colonel (Sir Frederick) Adam having been detached to occupy a post at Ordal, ten miles in advance of the army under Lord William Ben tinck, in Catalonia, was suddenly attacked by the French army, and his troops dispersed. This misfortune is ascribed to neglect in not having placed outposts, by which warning might be obtained of the enemy's approach.
An army which gains an advantage over its adversary is always more or less deranged by the action, and it is necessary that it should endea vour to recover its order preparatory to receiving the second line of the enemy, should the latter advance to renew the combat. On the enemy retiring, the first line of the victorious army advances, and then the second line follows it in order to support it, sending, if necessary, battalions or squadrons to replace such as have been most disordered during the action. In the event of the second line, or reserve, of the enemy being defeated, since then there is no apprehension that the action will be continued, companies of troops may be detached in pur suit of the retreating army ; but every precaution should be taken to keep them within the support of the main body, and particularly to prevent the troops from dispersing for the purpose of plundering the country. The advance of the whole army in pursuit should continue
so long only as it can be conducted with order, and in masses strong enough to oppose the enemy if his troops should rally in a good position. If disorder should take place among the pursuers, the latter should be made to fall back on the reserves : the pursuit of a retreating army can, indeed, be seldom continued beyond the first ele vated ground at which the latter may arrive ; since, however little discipline it may preserve, it may there rally and return to the order of battle. The consequences of the actions at Jena and Waterloo are exceptions to this rule, because the vanquished armies were at those places too completely disorganised to allow them to make any attempt to rally.
When the success of an action begins to be doubtful, and it is appre hended that the army must retreat, some of the heaviest artillery should be drawn off to a good position on heights, or behind streams or hollow ways, while the lighter artillery remains engaged. The first line of the defeated troops is then made to pass through the intervals of the second, or of the reserve, while the latter continues the action. The first line should remain in order of battle in rear of the second, till the Latter is enabled to retire ; and this alternate retreat of the lines should be continued till the army can be thrown into columns of march, when the retreat may be protected by detachments of light troops. In general the retreat should be made in one body, as thus it can more easily protect itself against the enemy in pursuit. If, how ever, the centre is broken, the army may be obliged to retire by differ ent and even by diverging routes ; and, provided there are in the rear strong posts by which it may be protected, the risk of being cut off during such a retreat is smalL When there are narrow defiles in rear of the field of battle, the retreat through them becomes extremely dangerous, for the army may be overtaken before it can get through ; and if they are already occu pied by the enemy's detachments, the retiring army may be annihi lated or compelled to surrender. It has been observed that the situation of the British army at Waterloo would have been very critical if it had been compelled to retreat ; this criticism, however, is hardly just. The British army had good roads to retire by, and the open forest in rear would have been an admirable position for defending the rear of the retreating columns. In order to pass a defile in safety, it ought to be previously occupied by troops : artillery and a reserve corps should also be stationed so as to defend the approaches on the advance of the enemy towards them.