Carl Clausewitz - The Art of Strategy - Napoleon's Maxims of War + Clausewitz's On War

After having selected an advantageous situation, Prince Eugene erected a battery of twenty pieces of cannon on a position which commanded the entire of the opposite bank, and covered his infantry by a line of entrenched parallels constructed on the slope of the declivity.

They were working vigorously at the bridge, when the Duke de Vendome appeared with his whole army. At first he seemed determined to oppose its construction, but after having examined the position of Prince Eugene, he judged this to be impracticable.

He therefore placed his army out of reach of the prince’s batteries, resting both his wings upon the river, so as to form a bow, of which the Adda was the cord. He then covered himself with entrenchments and abattis, and was thus enabled to charge the enemy’s columns whenever they debouched from the bridge, and to beat them in detail.

Eugene, having reconnoitred the position of the French, considered the passage impossible. He therefore withdrew the bridge, and broke up his camp during the night.

It was by this manœuvre, also, that, in the campaign of 1809, the Archduke Charles compelled the French to reoccupy the isle of Lobau, after having debouched on the left bank of the Danube. The march of the Archduke Charles was wholly concentric. He menaced Grosaspern with his right, Esling with his centre, and Enzersdorf with his left.

His army, with both wings resting on the Danube, formed a semicircle around Esling. Napoleon immediately attacked and broke the centre of the Austrians; but after having forced their first line, he found himself arrested by the reserves. In the meantime, the bridges upon the Danube had been destroyed, and several of his corps, with their parks of artillery, were still on the right bank. This disappointment, joined to the favorable position of the Austrians, decided Napoleon to re-enter the isle of Lobau, where he had previously constructed a line of field-works, so as to give it all the advantages of a well entrenched camp.

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It is difficult to prevent an enemy, supplied with pontoons, from crossing a river. When the object of an army, which defends the passage, is to cover a siege, the moment the general has ascertained his inability to oppose the passage, he should take measures to arrive before the enemy, at an intermediate position between the river he defends and the place he desires to cover.

Here we may observe, that this intermediate position should be reconnoitred, or rather, well entrenched beforehand; for the enemy will be unable to make an offensive movement against the corps employed in the siege, until he has beaten the army of observation; and the latter, under cover of its camp, may always await a favorable opportunity to attack him in flank or in rear.

Besides, the army which is once entrenched in this manner, has the advantage of being concentrated; while that of the enemy must act in detachments, if he wishes to cover his bridge, and watch the movements of the army of observation, so as to enable him to attack the besieging corps in its lines, without being exposed to an attempt on his rear, or being menaced with the loss of his bridge.

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In the campaign of 1645, Turenne was attacked with his army before Philipsburg by a very superior force. There was no bridge here over the Rhine, but he took advantage of the ground between the river and the place to establish his camp. This should serve as a lesson to engineer officers, not merely in the construction of fortresses, but of têtes de pont . A space should always be left between the fortress and the river, where an army may form and rally without being obliged to throw itself into the place, and thereby compromise its security. An army retiring upon Mayence before a pursuing enemy, is necessarily compromised; for this reason, because it requires more than a day to pass the bridge, and because the lines of Cassel are too confined to admit an army to remain there without being blocked up. Two hundred toises should have been left between that place and the Rhine. It is essential that all têtes de pont before great rivers should be constructed upon this principle, otherwise they will prove a very inefficient assistance to protect the passage of a retreating army. Têtes de pont , as laid down in our schools, are of use only for small rivers, the passage of which is comparatively short.

Marshal Saxe, in the campaign of 1741, having passed the Moldau in quest of a detached corps of fourteen thousand men, which was about to throw itself into Prague, left a thousand infantry upon that river, with orders to entrench themselves upon a height directly opposite the tête de pont . By this precaution, the marshal secured his retreat, and also the facility of repassing the bridge without disorder, by rallying his divisions between the entrenched height and the tête de pont .

Were these examples unknown to the generals of modern times, or are they disposed to think such precautions superfluous?

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Fortresses are equally useful in offensive and defensive warfare. It is true, they will not in themselves arrest an army, but they are an excellent means of retarding, embarrassing, weakening and annoying a victorious enemy.

The brilliant success of the allied armies in the campaign of 1814, has given to many military men a false idea of the real value of fortresses.

The formidable bodies which crossed the Rhine and the Alps at this period, were enabled to spare large detachments to blockade the strong places that covered the frontiers of France, without materially affecting the numerical superiority of the army which marched upon the capital. This army was in a condition, therefore, to act, without the fear of being menaced in its line of retreat.

But at no period of military history were the armies of Europe so combined before, or governed so entirely by one common mind in the attainment of a single object. Under these circumstances, the line of fortresses which surround France was rendered unavailable during the campaign; but it would be very imprudent, therefore, to conclude that a frontier guarded by numerous fortresses may be passed with impunity; or that battles may be fought with these places in your rear, without previously besieging, or at least investing them with sufficient forces.

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There are only two ways of insuring the success of a siege. The first, to begin by beating the enemy’s army employed to cover the place, forcing it out of the field, and throwing its remains beyond some great natural obstacle, such as a chain of mountains, or large river. Having accomplished this object, an army of observation should be placed behind the natural obstacle, until the trenches are finished and the place taken.

But if it be desired to take the place in presence of a relieving army, without risking a battle, then the whole materiel and equipment for a siege are necessary to begin with, together with ammunition and provisions for the presumed period of its duration, and also lines of contravallation and circumvallation, aided by all the localities of heights, woods, marshes and inundations.

Having no longer occasion to keep up communications with your depôts, it is now only requisite to hold in check the relieving army. For this purpose, an army of observation should be formed, whose business it is never to lose sight of that of the enemy, and which, while it effectually bars all access to the place, has always time enough to arrive upon his flanks or rear in case he should attempt to steal a march.