1. The besieging, blockade, and investment of fortresses, generally increase as the army advances. This weakening influence alone acts so powerfully on the condition of the combatant force, that it may soon outweigh all the advantages gained. No doubt, in modern times, a system has been introduced of blockading places with a small number of troops, or of watching them with a still smaller number; and also the enemy must keep garrisons in them. Nevertheless, they remain a great element of security. The garrisons consist very often in half of people, who have taken no part in the war previously. Before those places which are situated near the line of communication, it is necessary for the assailant to leave a force at least double the strength of the garrison; and if it is desirable to lay formal siege to, or to starve out, one single considerable place, a small army is required for the purpose.
2. The second cause, the taking up a theatre of war in the enemy’s country, increases necessarily with the advance, and if it does not further weaken the condition of the combatant force at the moment, it does so at all events in the long run.
We can only regard as our theatre of war, so much of the enemy’s country as we actually possess; that is to say, where we either have small corps in the field, or where we have left here and there strong garrisons in large towns, or stations along the roads, etc.; now, however small the garrisons may be which are detached, still they weaken the combatant force considerably. But this is the smallest evil.
Every army has strategic flanks, that is, the country which borders both sides of its lines of communications; the weakness of these parts is not sensibly felt as long as the enemy is similarly situated with respect to his. But that can only be the case as long as we are in our own country; as soon as we get into the enemy’s country, the weakness of these parts is felt very much, because the smallest enterprise promises some result when directed against a long line only feebly, or not all, covered; and these attacks may be made from any quarter in an enemy’s country.
The further we advance, the longer these flanks become, and the danger arising from them is enhanced in an increased ratio, for not only are they difficult to cover, but the spirit of enterprise is also first roused in the enemy, chiefly by long insecure lines of communication, and the consequences which their loss may entail in case of a retreat are matter of grave consideration.
All this contributes to place a fresh load on an advancing army at every step of its progress; so that if it has not commenced with a more than ordinary superiority, it will feel itself always more and more cramped in its plans, gradually weakened in its impulsive force, and at last in a state of uncertainty and anxiety as to its situation.
3. The third cause, the distance from the source from which the incessantly diminishing combatant force is to be just as incessantly filled up, increases with the advance. A conquering army is like the light of a lamp in this respect; the more the oil which feeds it sinks in the reservoir and recedes from the focus of light, the smaller the light becomes, until at length it is quite extinguished.
The richness of the conquered provinces may certainly diminish this evil very much, but can never entirely remove it, because there are always a number of things which can only be supplied to the army from its own country, men in particular; because the subsidies furnished by the enemy's country are, in most cases, neither so promptly nor so surely forthcoming as in our own country; because the means of meeting any unexpected requirement cannot be so quickly procured; because misunderstandings and mistakes of all kinds cannot so soon be discovered and remedied.
If a prince does not lead his army in person, as became the custom in the last wars, if he is not anywhere near it, then another and very great inconvenience arises in the loss of time occasioned by communications backwards and forwards; for the fullest powers conferred on a commander of an army, are never sufficient to meet every case in the wide expanse of his activity.
4. The change in political alliances. If these changes, produced by a victory, should be such as are disadvantageous to the conqueror, they will probably be so in a direct relation to his progress, just as is the case if they are of an advantageous nature. This all depends on the existing political alliances, interests, customs, and tendencies, on princes, ministers, etc. In general, we can only say that when a great state which has smaller allies is conquered, these usually secede very soon from their alliance, so that the victor, in this respect, with every blow becomes stronger; but if the conquered state is small, protectors much sooner present themselves when his very existence is threatened, and others, who have helped to place him in his present embarrassment, will turn round to prevent his complete downfall.