Front Page Titles (by Subject) 2.—: DEFENSIVE - On War, vol. 3
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2.—: DEFENSIVE - Carl von Clausewitz, On War, vol. 3 
On War, trans. Col. J.J. Graham. New and Revised edition with Introduction and Notes by Col. F.N. Maude, in Three Volumes (London: Kegan Paul, Trench, Trubner & C., 1918). Vol. 3.
Part of: On War
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(1) In political language, a defensive War is one which a State carries on to maintain its independence: in Strategy, a defensive War is a campaign in which we limit ourselves to contending with the enemy in a theatre of War which has been prepared by us for the purpose. Whether the battles we fight in this theatre of War are offensive or defensive makes no difference in this respect.
(2) We choose the strategic defensive chiefly when the enemy is superior in force. Naturally fortresses and entrenched camps, which are to be regarded as the chief preparations of a theatre of War, afford great advantages, to which may be added knowledge of the country and the possession of good maps and surveys. With these advantages, a small Army, or an Army which is based on a small State and limited resources, will be more in a condition to oppose the enemy than without the aid of such assistance.
There are besides the two following grounds upon which we may choose the defensive form of War by preference:
First.—If the poverty of the provinces surrounding our theatre of War makes our operations extremely difficult on account of the question of subsistence. In that case we escape the disadvantage, and the enemy must submit to it. This is, for instance, at this moment (1812) the case of the Russian Army.
Secondly.—If the enemy has greater advantages for carrying on the War. In a prepared theatre of War—which we know, where all the surrounding circumstances are in our favour—War is more easily conducted; there will not be so many faults committed. In this case, that is, when the little dependence to be placed on our troops and Generals compels us to resort to the defensive, we gladly combine the tactical defensive with the strategic, that is, we give battle in positions prepared beforehand; we do so further because there is less risk of our committing faults.
(3) In defensive War, just as much as in the offensive, a great object should be pursued. This can be nothing else than to annihilate the enemy’s Army, either in a battle, or by making his subsistence so difficult as to produce disorganisation and compel him to retreat, by which he must necessarily suffer considerable losses. Wellington’s campaign in the years 1810 and 1811 is an instance of this.
The defensive War, therefore, does not consist in an indolent waiting for events; we must only pursue the waiting-for system where there is a palpable and decisive utility in that mode of procedure. That sort of calm before a storm, whilst the offensive is gathering up new force for great blows, is extremely dangerous for the defender.
If the Austrians, after the battle of Aspern, had reinforced themselves to three times the strength of the French Emperor, which they certainly might have done, then the time of rest which took place before the battle of Wagram might have been advantageous to them, but only on that condition; as they did not do so, it was so much lost time for them, and it would have been wiser if they had taken advantage of Napoleon’s critical position to reap the fruits of their success at Aspern.
(4) Fortresses are intended to occupy an important part of the enemy’s Army in besieging them. This period must, therefore, be taken advantage of to beat the rest of the Army. Our battles should be fought behind our fortresses, not in front of them. At the same time, however, we must not quietly look on at their being captured, as Benningsen did during the siege of Dantzig.
(5) A great river, that is, one we cannot build a bridge across without considerable difficulty—rivers like the Danube below Vienna, and the Lower Rhine—affords a natural line of defence of which we can avail ourselves, not by distributing our forces equally along its banks, and seeking to hinder the passage absolutely, which is a dangerous measure, but by watching it, and when the enemy passes, then falling upon him from all sides just at the moment when he has not yet got all his forces under command, and is still hemmed in within a narrow space close to the river. The battle of Aspern is an instance. At the battle of Wagram the Austrians, without any necessity, allowed the French to get possession of far too much space, by which means they did away with the disadvantages peculiarly inherent to the passage of a river.
(6) Mountains are the second natural obstacles of ground which afford a good line of defence, as we can either have them in front, and only occupy them with a few light troops, treat them to a certain extent as a river which the enemy must cross, and as soon as he debouches with his single columns, fall upon one of them with our whole weight, or we may ourselves take position in the mountains. In the last case, we must only defend the single passes with small detachments, and a considerable part of the Army (a third or a half) must remain in reserve, in order to fall in superior numbers on any column which forces its way through. This great reserve must, however, not be split up with a view to absolutely preventing all the columns from passing, but we must, from the first, resolve to make use of it to attack that column which we suppose to be the strongest. If, in this way, we rout a considerable part of the enemy’s force, the other columns which have forced their way through will of themselves retire again.
The formation of mountain ranges in general is such that about the centre of the masses there are plateaux or plains at a greater or less elevation, and the sides next to the level country are intersected by deep valleys forming the entrances or avenues. The defender, therefore, has in the mountains a district in which he can make rapid movements right or left, whilst the attacking columns are separated from each other by steep, inaccessible ridges. It is only a mountain mass of this kind that is well adapted for a good defence. If it is rugged and impassable generally throughout, so that the Corps on the defensive must be scattered and disconnected, then to undertake the defence with the principal Army is a dangerous measure, for under such circumstances all the advantages are on the side of the assailant, who can fall upon any of the isolated posts with far superior numbers, as no pass, no single post is so strong that it cannot soon be taken by superior numbers.
(7) With regard to mountain warfare, it is specially to be observed that in it a great deal depends on the aptitude of subordinate officers, but still more on the high spirit which animates the ranks. Great skill in manœuvring is not here requisite, but a military spirit and a heart in the cause, for every one is more or less left to act independently; this is why national levies find their account in mountain warfare, for while they are deficient in the first quality, they possess the other in the highest degree.
(8) Lastly, in respect to the strategic defensive, it is to be observed that, while it is in itself stronger than the offensive, it should only be used to gain the first great result, and that if this object is attained, and peace does not immediately follow upon that, greater results can only be obtained by the offensive; for whoever remains always on the defensive exposes himself to the disadvantage of always carrying on the War at his own expense. No State can endure that for more than a certain time; and therefore, if it exposes itself to the blows of its adversary without ever striking in return, it is almost sure in the end to become exhausted, and be obliged to submit. We should therefore begin with the defensive, that we may with the more certainty end with the offensive.