For the same frontier it is objectionable to form more than one army; grand detachments and corps of observation may frequently be used with advantage, but double or multiple lines of operation are far less favorable than one simple line. It may however sometimes occur that the position of the enemy's forces will be such as to make this operation the preferable one. In that case, interior lines should always be adopted, unless we have a vast superiority in number. Double exterior lines, with corps several days' march asunder, must be fatal, if the enemy, whether acting on single or double interior lines, take advantage of his position to concentrate his masses successively against our isolated forces. The Roman armies under the consuls Flaminius and Servilius opposed Hannibal on exterior lines, the one by Florence and Arrezzio, and the other by Modena and Ariminum. Hannibal turned the position of Flaminius and attacked the Roman armies separately, gaining a complete and decisive victory. Such also was the character of the operations of the French in 1795, under Pichegru and Jourdan; they met with a bloody and decisive defeat. Again in 1796, the French armies under Jourdan and Moreau, pursued exterior lines; the Archduke Charles, from his interior position, succeeded in defeating both the opposing generals, and forcing them to retreat. If the two armies united had pursued a single line, the republican flag had been carried in triumph to Vienna.
Converging lines of operation are preferable, under most circumstances, to diverging lines. Care should be taken, however, that the point of meeting be such that it may not be taken as a strategic position by the enemy, and our own forces be destroyed in detail, before they can effect a junction. In 1797 the main body of the Austrians, under Alvinzi, advanced against Napoleon, on three separate lines, intending to concentrate at Rivoli, and then attack the French in mass; but Napoleon took his strategic position at Rivoli, and overthrew the enemy's corps as they successively appeared. In the same way the Archduke Charles took an interior position, between Moreau and Jourdan, in 1796, and prevented them from concentrating their forces on a single point. Wurmser and Quasdanowich attempted to concentrate their forces on the Mincio, by moving on the opposite shores of Lake Garda; but Napoleon took an interior position and destroyed them. In 1815 Blucher and Wellington, from their interior position, prevented the junction of Napoleon and Grouchy.
Diverging lines may be employed with advantage against an enemy immediately after a successful battle or strategic manœuvre; for by this means we separate the enemy's forces, and disperse them; and if occasion should require it, may again concentrate our forces by converging lines. Such was the manœuvre of Frederick the Great, in 1757, which produced the battles of Rosbach and Leuthen; such also was the manœuvre of Napoleon at Donawert in 1805, at Jena in 1806, and at Ratisbon in 1809.
Interior lines of operations, when properly conducted, have almost invariably led to success: indeed every instance of failure may be clearly traced to great unskilfulness in their execution, or to other extraneous circumstances of the campaign. There may, however, be cases where it will be preferable to direct our forces on the enemy's flank; the geographical character of the theatre of war, the position of other collateral forces, &c., rendering such a direction necessary. But as a general rule, interior and central lines, for an army of moderate forces, will lead to decisive results.
Napoleon's Italian campaigns in 1796 and 1797, the campaign of the Archduke Charles in 1796, Napoleon's campaigns of 1805 and 1809 against Austria, and of 1806 and 1807 against Prussia and Russia, of 1808 in Spain, his manœuvres in 1814, between the battle of Brienne and that of Paris, and his operations previous to the Battle of Ligny in 1815, are all brilliant examples under this head.
To change the line of operations, in the middle of a campaign, and follow accidental lines, is always a delicate affair, and can only be resorted to by a general of great skill, and with disciplined troops. In such a case it may be attended with important results. It was one of Napoleon's maxims, that "a line of operations, when once chosen, should never be abandoned." This maxim, however, must sometimes be disregarded by an army of undisciplined troops, in order to avoid entire destruction; but the total abandonment of a line of operations is always attended with great loss, and should be regarded as a mere choice of evils. A regular army can always avoid this result, by changing the direction of its line; thus frequently gaining superior advantages in the new theatre of action. If the plan of this change be the result of a good coup d'oeil, and it be skilfully executed, the rear of the operating army will be secure from the enemy; and moreover, he will be left in doubt respecting its weak points. But such is the uncertainty of this manœuvre, that it is very rarely taken by the best troops, unless actually forced upon them. If the army be of incongruous materials, generally a change of direction will be less advantageous than to entirely abandon the line, and save as many as possible of the troops for some new plan of operations. (Maxim 20.) If, however, the undisciplined army be sustained by fortifications, it can take up the accidental line of operations in the same manner, and with the same probability of success, as is done by a regular force.
We have examples of accidental lines in the operations of the king of Prussia, after the battle of Hohenkirchen, and of Washington, in New-Jersey, after the action of Princeton. This is one of the finest in military history. Napoleon had projected a change in his line of operations, in case he lost the battle of Austerlitz; but victory rendered its execution unnecessary. Again in 1814 he had planned an entire change of operations; but the want of co-operation of the forces under Mortier and Marmont forced him to abandon a plan which, if properly executed, had probably defeated the allies. Jomini pronounced it one of the most brilliant of his military career.
Having explained the principal terms used in strategy, let us trace out the successive operations of war in their usual strategic relations.
We will suppose war to be declared, and the army to be just entering upon a campaign. The political and military authorities of the state determine upon the nature of the war, and select the theatre of its enterprises. The chief selects certain points, on or near the borders of the seat of war, where his troops are to be assembled, and his materiel collected. These points, together, form his base of operations. He now selects some point, within the theatre of the war, as the first object of his enterprises, and chooses the line of operations most advantageous for reaching this objective point. The temporary positions taken on this line become strategic positions, and the line in his rear, a line of defence. When he arrives in the vicinity of his first object, and the enemy begins to oppose his enterprises, he must force this enemy to retreat, either by an attack or by manœuvres. For this purpose he temporarily adopts certain lines of manœuvre, which may deviate from his general line of operations. The ulterior events of the campaign may possibly cause him to make these new, or accidental lines, his lines of operations. The approach of hostile forces may cause him to detach secondary corps on secondary lines; or to divide his army, and pursue double or multiple lines. The primitive object may also be relinquished, and new ones proposed, with new lines and new plans of operations. As he advances far from his primitive base, he forms new dépôts and lines of magazines. He may encounter natural and artificial obstacles. To cross large rivers in the face of an enemy is a hazardous operation; and he requires all the art of the engineer in constructing bridges, and securing a safe passage for his army. If a fortified place is to be taken, he will detach a siege corps, and either continue his march with the main army, or take a strategic position to cover this siege. Thus Napoleon, in 1796, with an army of only 50,000 combatants, could not venture to penetrate into Austria, with Mantua and its garrison of 25,000 men in his rear, and an Austrian force of 40,000 before him. But in 1806 the great superiority of his army enabled him to detach forces to besiege the principal fortresses of Silesia, and still to continue his operations with his principal forces. The chief of the army may meet the enemy under circumstances such as to induce or compel him to give battle. If he should be victorious, the enemy must be pursued and harassed to the uttermost. If he should be defeated, he must form the best plan, and provide the best means of retreat. If possible, he must take shelter in some line of fortifications,