Convoy.—­A convoy consists of provisions, military munitions, &c., sent from one point to another, under the charge of a detachment of troops, called an escort.  When regular depots and magazines are established, with proper relations to the line of operations, convoys requiring particular escorts are seldom necessary, because the position of the army will cover the space over which the magazines are to be moved.  But in the immediate vicinity of the enemy, or in a country whose inhabitants are hostile or insurrectionary, precautions of this kind should always be resorted to.

The size and composition of the escort must depend upon the nature of the country and the imminence of the danger.  The ground to be passed over should be previously reconnoitred, and the line of march be taken up only after the most satisfactory reports.  When once put in motion, the convoy should be thoroughly hemmed in by flankers, to give warning to the escort of the approach of the enemy.  Small parties of cavalry are detached on all sides, but particularly in advance.  The main body of the escort is concentrated on the most exposed point of the convoy while the other sides are guarded by subdivisions.  In case of an attack by a large party, the baggage wagons may be formed into a kind of defensive field-work, which, with one or two pieces of light artillery, can in this way resist a pretty strong effort to destroy or carry away the convoy.

As a general rule, it is better to supply the wants of an army by small successive convoys than by periodical and large ones.  Even should some of the former be captured their loss would not be materially felt; but a large periodical convoy offers so great a temptation to the enterprise of the enemy, and is so difficult to escort, that he will venture much to destroy it, and its loss may frustrate our plans of a siege or of an important military operation.  If the Prussian army, when besieging Olmutz, had observed this rule, the capture of a convoy would not have forced them to raise the siege and to retreat.

Napoleon estimates that an army of 100,000 men in position will require the daily arrival of from four to five hundred wagon loads of provisions.

The difficulty of moving provisions, baggage, &c., in a retreat, is always very great, and the very best generals have frequently failed on this point.  Indeed, the best concerted measures will sometimes fail, amid the confusion and disorder consequent upon a retreat with an able and active enemy in pursuit.  In such a case, the loss of the provision-trains in a sterile or unfriendly country may lead to the most terrible disasters.  We will allude to two examples of this kind:  the retreat of the English from Spain in 1809, and that of the French from Russia in 1812.