The Law of Nations or the Principles of Natural Law (1758)

Emmerich de Vattel

Of the Enemy’s Allies: of Warlike Associations, of Auxiliaries and Subsidies

§ 78. Treaties relative to war.
We have sufficiently spoken of treaties in general, and shall here touch on this subject only in its particular relations to war. Treaties relating to war are of several kinds, and vary in their objects and clauses, according to the will of those who make them. Besides applying to them all that we have said of treaties in general (Book II. Ch. XII. etc.), they may also be divided into treaties real and personal, equal and unequal, etc. But they have also their specific differences, viz. those which relate to their particular object, war.

§ 79. Defensive and offensive alliances.
Under this relation, alliances made for warlike purposes are divided in general into defensive and offensive alliances. In the former, the nation engages only to defend her ally in case he be attacked: in the latter, she unites with him for the purpose of making an attack, — of jointly waging war against another nation. Some alliances are both offensive and defensive; and there seldom is an offensive alliance which is not also a defensive one. But it is very usual for alliances to be purely defensive: and these are in general the most natural and lawful. It would be a tedious and even a useless task to enumerate in detail all the varieties incident to such alliances. Some are made, without restriction, against all opponents: in others, certain states are excepted: others again are formed against such or such a nation expressly mentioned by name.

§ 80. Difference between warlike associations and auxiliary treaties.
But a difference of great importance to be observed, especially in defensive alliances, is that between an intimate and complete alliance, in which we agree to a union of interests, — and another, in which we only promise a stated succor. The alliance in which we agree to a union of interests is a warlike association: each of the parties acts with his whole force; all the allies become principals in the war, they have the same friends and the same enemies. But an alliance of this nature is more particularly termed a warlike association, when it is offensive.

§ 81. Auxiliary troops.
When a sovereign, without directly taking part in the war made by another sovereign, only sends him succors of troops or ships, these are called auxiliaries.

The auxiliary troops serve the prince to whom they are sent, according to their sovereign’s orders. If they are purely and simply sent without restriction, they are to serve equally on the offensive and the defensive; and for the particulars of their operations, they are to obey the directions of the prince to whose assistance they come. Yet this prince has not the free and entire disposal of them, as of his own subjects: they are granted to him only for his own wars; and he has no right to transfer them, as auxiliaries, to a third power.

§ 82. Subsidies.
Sometimes, this succor from a potentate who does not directly take part in the war, consists in money; and then it is called a subsidy. This term is now often taken in another sense, and signifies a sum of money annually paid by one sovereign to another, in return for a body of troops which the latter furnishes to the other to carry on his wars, or keeps in readiness for his service. The treaties for procuring such a resource are called subsidiary treaties. France and England have at present such treaties existing with several of the northern powers and princes in Germany, and continue them even in times of peace.

§ 83. When a nation is allowed to assist another.
In order, now, to judge of the morality of these several treaties or alliances, — of their legitimacy according to the law of nations, we must, in the first place, lay down this incontrovertible principle, that It is lawful and commendable to succor and assist, by all possible means, a nation engaged in a just war; and it is even a duty incumbent on every nation, to give such assistance, when she can give it without injury to herself. But no assistance whatever is to be afforded to him who is engaged in an unjust war. There is nothing in this which is not demonstrated by what we have said of the common duties of nations towards each other. (Book II. Ch. I.) To support the cause of justice when we are able, is always commendable: but, in assisting the unjust, we partake of his crime, and become, like him, guilty of injustice.

§ 84. and to make alliances for war.
If, to the principle we have now laid down, you add the consideration of what a nation owes to her own safety, and of the care which it is so natural and so fit that she should take to put herself in a condition to resist her enemies, you will the more readily perceive how clear a right a nation has to make warlike alliances, and especially defensive alliances, whose sole tendency is to maintain all parties in the quiet and secure possession of their property.

But great circumspection is to be used in forming such alliances. Engagements by which a nation maybe drawn into a war at a moment when she least expects it, ought not to be contracted without very important reasons, and a direct view to the welfare of the state. We here speak of alliances made in time of peace, and by way of precaution against future contingencies.

§ 85. Alliances made with a nation actually engaged in war.
If there be question of contracting an alliance with a nation already engaged in a war, or on the point of engaging in one, two things are to be considered: 1. The justice of that nation’s quarrel. 2. The welfare of the state. If the war which a prince wages, or is preparing to wage, be unjust, it is not allowable to form an alliance with him; for injustice is not to be supported. If he is justifiable in taking up arms, it still remains to be considered whether the welfare of the state allows or requires us to embark in his quarrel: for it is only with a view to the welfare of the state that the sovereign ought to use his authority: to that all his measures should tend, and especially those of the most important nature. What other consideration can authorize him to expose his people to the calamities of war?

§ 86. Tacit clause in every warlike alliance.
As it is only for the support of a just war that we are allowed to give assistance or contract alliances, — every alliance, every warlike association, every auxiliary treaty, contracted by way of anticipation in time of peace, and with no view to any particular war, necessarily and of itself includes this tacit clause — that the treaty shall not be obligatory except in case of a just war. On any other footing, the alliance could not be validly contracted. (Book II. §§ 161, 168.)

But care must be taken that treaties of alliance be not thereby reduced to empty and delusive formalities. The tacit restriction is to be understood only of a war which is evidently unjust; for otherwise a pretense for eluding treaties would never be wanting. Is there question of contracting an alliance with a power actually at war? It behooves you most religiously to weigh the justice of his cause: the judgment depends solely on you, since you owe him no assistance any further than as his quarrel is just, and your own circumstances make it convenient for you to embark in it. But when once engaged, nothing less than the manifest injustice of his cause can excuse you from assisting him. In a doubtful case, you are to presume that your ally has justice on his side; that being his concern.

But if you entertain strong doubts, you may very fairly and commendably interpose to effect an accommodation. Thus you may bring the justice of the cause to the test of evidence, by discovering which of the contending parties refuses to accede to equitable conditions.

§ 87. To refuse succors for an unjust war is no breach of alliance.
As every alliance implies the tacit clause above mentioned, he who refuses to succor his ally in a war that is manifestly unjust is not chargeable with a breach of alliance.

§ 88. What the casus fœderis is.
When alliances have thus been contracted beforehand, the question is, to determine, in the course of events, those cases in which our engagements come in force, and we are bound to act in consequence of the alliance. This is what is called casus fœderis, or case of the alliance, and is to be discovered in the concurrence of the circumstances for which the treaty has been made, whether those circumstances have been expressly specified in it, or tacitly supposed. Whatever has been promised in the treaty of alliance is due in the casus fœderis, and not otherwise.

§ 89. It never takes place in an unjust war.
As the most solemn treaties cannot oblige any one to favor an unjust quarrel (§ 86): the casus fœderis never takes place in a war that is manifestly unjust.

§ 90. How it exists in a defensive war.
In a defensive alliance, the casus fœderis does not exist immediately on our ally being attacked. It is still our duty to examine whether he has not given his enemy just cause to make war against him: for we cannot have engaged to undertake his defense with the view of enabling him to insult others, or to refuse them justice. If he is in the wrong, we must induce him to offer a reasonable satisfaction; and if his enemy will not be contented with it, then, and not till then, the obligation of defending him commences.

§ 91. and in a treaty of guarantee.
But if the defensive alliance contains a guarantee of all the territories at that time possessed by the ally, the casus fœderis immediately takes place whenever those territories are invaded or threatened with an invasion. If they are attacked for a just cause, we must prevail on our ally to give satisfaction; but we may on good grounds oppose his being deprived of his possessions, as it is generally with a view to our own security that we undertake to guaranty them. On the whole, the rules of interpretation, which we have given in an express chapter,1 are to be consulted, in order to determine, on particular occasions, the existence of the casus fœderis.

§ 92. The succor is not due under an inability to furnish it, or when the public safetyt would be exposed.
If the state that has promised succors finds herself unable to furnish them, her inability alone is sufficient to dispense with the obligation; and if she cannot give her assistance without exposing herself to evident danger, this circumstance also dispenses with it.

This would be one of those cases in which a treaty becomes pernicious to the state, and therefore not obligatory (Book II. § 160). But we here speak of an imminent danger, threatening the very existence of the state. The case of such a danger is tacitly and necessarily reserved in every treaty. As to remote dangers, or those of no extraordinary magnitude, — since they are inseparable from every military alliance, it would be absurd to pretend that they should create an exception; and the sovereign may expose the nation to them in consideration of the advantages which she reaps from the alliance.

In virtue of these principles, we are absolved from the obligation of sending assistance to an ally while we are ourselves engaged in a war which requires our whole strength. If we are able to oppose our own enemies and to assist our ally at the same time, no reason can be pleaded for such dispensation. But, in such cases, it rests with ourselves to determine what our circumstances and strength will allow. It is the same with other things which may have been promised, as, for instance, provisions. There is no obligation to furnish an ally with them when we want them for our own use.

§ 93. Other cases.
We forbear to repeat in this place what we have said of various other cases, in discoursing of treaties in general, as, for example, of the preference due to the more ancient ally (Book II. § 167), and to a protector (ibid. § 204), of the meaning to be annexed to the term “allies,” in a treaty in which they are reserved (ibid. § 309). Let us only add, on this last question, that, in a warlike alliance made against all opponents, the allies excepted, this exception is to be understood only of the present allies. Otherwise, it would afterwards be easy to elude the former treaty by new alliances; and it would be impossible for us to know either what we are doing in concluding such a treaty, or what we gain by it.

A case which we have not spoken of is this: — Three powers have entered into a treaty of defensive alliance: two of them quarrel, and make war on each other: — how is the third to act? The treaty does not bind him to assist either the one or the other; for it would be absurd to say that he has promised his assistance to each against the other, or to one of the two in prejudice of the other. The only obligation, therefore, which the treaty imposes on him, is to endeavor, by the interposition of his good offices, to effect a reconciliation between his allies; and if his mediation proves unsuccessful, he remains at liberty to assist the party who appears to have justice on his side.

§ 94. Refusal of the succors due in virtue of an alliance.
To refuse an ally the succors due to him, without having any just cause to allege for such refusal, is doing him an injury, since it is a violation of the perfect right which we gave him by a formal engagement. I speak of evident cases, it being then only that the right is perfect; for, in those of a doubtful nature, it rests with each party to judge what he is able to do (§ 92): but he is to judge maturely and impartially, and to act with candor. And as it is an obligation naturally incumbent on us, to repair any damage caused by our fault, and especially by our injustice, we are bound to indemnify an ally for all the losses he may have sustained in consequence of our unjust refusal. How much circumspection, therefore, is to be used in forming engagements, which we cannot refuse to fulfill without material injury to our affairs or our honor, and which, on the other hand, if complied with, may be productive of the most serious consequences.

§ 95. The enemy’s associates.
An engagement, which may draw us into a war, is of great moment: in it the very existence of the state is at stake. He who in an alliance promises a subsidy or a body of auxiliaries, sometimes imagines that he only risks a sum of money or a certain number of soldiers; whereas he often exposes himself to war and all its calamities. The nation against whom he furnishes assistance will look upon him as her enemy; and should her arms prove successful, she will carry the war into his country. But it remains to be determined whether she can do this with justice, and on what occasions. Some authors2 decide in general, that whoever joins our enemy, or assists him against us with money, troops, or in any other manner whatever, becomes thereby our enemy, and gives us a right to make war against him: — a cruel decision, and highly inimical to the peace of nations! It cannot be supported by principles; and happily the practice of Europe stands in opposition to it.

It is true, indeed, that every associate of my enemy is himself my enemy. It is of little consequence whether any one makes war on me directly, and in his own name, or under the auspices of another. Whatever rights war gives me against my principal enemy, the like it gives me against all his associates: for I derive those rights from the right to security, — from the care of my own defense; and I am equally attacked by the one and the other party. But the question is, to know whom I may lawfully account my enemy’s associate, united against me in war.

§ 96. Those who make a common cause with the enemy are his associates
First, in that class I shall rank all those who are really united in a warlike association with my enemy, and who make a common cause with him, though it is only in the name of that principal enemy that the war is carried on. There is no need of proving this. In the ordinary and open warlike associations, the war is carried on in the name of all the allies, who are equally enemies (§ 80).

§ 97. And those who assist him, without being obliged to it by treaties.
In the second place, I account as associates of my enemy, those who assist him in his war without being obliged to it by any treaty. Since they freely and voluntarily declare against me, they, of their own accord, choose to become my enemies. If they go no farther than furnishing a determined succor, allowing some troops to be raised, or advancing money, — and, in other respects, preserve towards me the accustomed relations of friendship and neutrality, — I may overlook that ground of complaint; but still I have a right to call them to account for it. This prudent caution of not always coming to an open rupture with those who give such assistance to our enemy, that we may not force them to join him with all their strength, — this forbearance, I say, has gradually introduced the custom of not looking on such assistance as an act of hostility, especially when it consists only in the permission to enlist volunteers. How often have the Switzers granted levies to France, at the same time that they refused such an indulgence to the house of Austria, though both powers were in alliance with them! How often have they allowed one prince to levy troops in their country, and refused the same permission to his enemy, when they were not in alliance with either! They granted or denied that favor according as they judged it most expedient for themselves; and no power has ever dared to attack them on that account. But if prudence dissuades us from making use of all our right, it does not thereby destroy that right, A cautious nation chooses rather to overlook certain points, than unnecessarily to increase the number of her enemies.

§ 98. Or who are in an offensive alliance with him.
Thirdly, those, who, being united with my enemy by an offensive alliance, actively assist him in the war which he declares against me, — those, I say, concur in the injury intended against me. They show themselves my enemies, and I have a right to treat them as such. Accordingly, the Switzers, whose example we have above quoted, seldom grant troops except for defensive war. To those in the service of France, it has ever been a standing order from their sovereigns, not to carry arms against the empire, or against the states of the house of Austria in Germany. In 1644, the captains of the Neufchatel regiment of Guy, on information that they were destined to serve under Marshal Turenne, in Germany, declared that they would rather die than disobey their sovereign and violate the alliances of the Helvetic body. Since France has been mistress of Alsace, the Switzers who serve in her armies never pass the Rhine to attack the empire. The gallant Daxelhoffer, captain of a Berne company in the French service, consisting of 200 men, and of which his four sons formed the first rank, seeing the general would oblige him to pass the Rhine, broke his espontoon, and marched back with his company to Berne.

§ 99. How a defensive alliance associates with the enemy.
Even a defensive alliance made expressly against me, or (which amounts to the same thing) concluded with my enemy during the war, or on the certain prospect of its speedy declaration, is an act of association against me; and if followed by effects, I may look on the party who has contracted it as my enemy. The case is here precisely the same as that of a nation assisting my enemy without being under any obligation to do so, and choosing of her own accord to become my enemy. (See § 97).

§ 100. Another case.
A defensive alliance, though of a general nature, and made before any appearance of the present war, produces also the same effect, if it stipulates the assistance of the whole strength of the allies: for in this case it is a real league, or warlike association; and, besides, it were absurd that I should be debarred from making war on a nation who opposes me with all her might, and thus exhausting the source of those succors with which she furnishes my enemy. In what light am I to consider an auxiliary who comes to make war on me at the head of all his forces? It would be mockery on his part, to pretend that he is not my enemy. What more could he do, were he openly to declare himself such? He shows no tenderness for me on the occasion: he only wishes that a tender regard should be paid to himself. And shall I suffer him to preserve his provinces in peace, and secure from all danger, whilst he is doing me all the mischief in his power? No! the law of nature, the law of nations, obliges us to be just: but does not condemn us to be dupes.

§ 101. In what case it does not produce the same effect.
But, if a defensive alliance has not been made against me in particular, nor concluded at the time when I was openly preparing for war, or had already begun it, — and if the allies have only stipulated in it that each of them shall furnish a stated succor to him who shall be attacked, — I cannot require that they should neglect to fulfill a solemn treaty, which they had an unquestionable right to conclude without any injury to me. In furnishing my enemy with assistance, they only acquit themselves of a debt: they do me no wrong in discharging it; and, consequently, they afford me no just grounds for making war on them (§ 26). Neither can I say that my safety obliges me to attack them; for I should thereby only increase the number of my enemies, and, instead of a slender succor which they furnish against me, should draw on myself the whole power of those nations. It is, therefore, only the troops which they send as auxiliaries, that I am to consider as enemies. These are actually united with my enemies and fighting against me.

The contrary principles would tend to multiply wars, and spread them beyond all bounds, to the common ruin of nations. It is happy for Europe, that, in this instance, the established custom is in accord with the true principles. A prince seldom presumes to complain of a nation’s contributing to the defense of her ally by furnishing him with succors which were promised in former treaties, — in treaties that were not made against that prince in particular. In the last war, the United Provinces long continued to supply the queen of Hungary with subsidies, and even with troops; and France never complained of these proceedings till those troops marched into Alsace to attack the French frontier. Switzerland, in virtue of her alliance with France, furnishes that crown with numerous bodies of troops, and, nevertheless, lives in peace with all Europe.

There is one case, however, which might form an exception to the general rule; it is that of a defensive war which is evidently unjust. For in such case there no longer exists any obligation to assist an ally (§§ 86, 87, 89). If you undertake to do it without necessity, and in violation of your duty, you do an injury to the enemy, and declare against him out of mere wantoness. But this is a case that very rarely occurs between nations. There are few defensive wars without at least some apparent reason to warrant their justice or necessity. Now, on any dubious occasion, each state is sole judge of the justice of her own cause; and the presumption is in favor of your ally (§ 86). Besides, it belongs to you alone to determine what conduct on your part will be conformable to your duties and to your engagements; and consequently nothing less than the most palpable evidence can authorize the enemy of your ally to charge you with supporting an unjust war, contrary to the conviction of your own conscience. In fine, the voluntary law of nations ordains, that, in every case susceptible of doubt, the arms of both parties shall, with regard to external effects, be accounted equally lawful (§ 40).

§ 102. Whether it be necessary to declare war against the enemy’s associates.
The real associates of my enemy being my enemies, I have against them the same rights as against the principal enemy (§ 95). And as their own conduct proclaims them my enemies, and they take up arms against me in the first instance, I may make war on them without any declaration: the war being sufficiently declared by their own act. This is especially the case of those who in any manner whatever concur to make an offensive war against me; and it is likewise the case of all those whom we have mentioned in §§ 96, 97, 98, 99, 100.

But it is not thus with those nations which assist my enemy in a defensive war: I cannot consider them as his associates (§ 101). If I am entitled to complain of their furnishing him with succors, this is a new ground of quarrel between me and them. I may expostulate with them, and, on not receiving satisfaction, prosecute my right, and make war on them. But in this case there must be a previous declaration (§ 51). The example of Manlius, who made war on the Galatians for having supplied Antiochus with troops, is not a case in point. Grotius3 censures the Roman general for having begun that war without a declaration. The Galatians, in furnishing troops for an offensive war against the Romans, had declared themselves enemies to Rome. It would appear, indeed, that, on peace being concluded with Antiochus, Manlius ought to have waited for orders from Rome before he attacked the Galatians; and then, if that expedition was considered as a fresh war, he should have not only issued a declaration, but also made a demand of satisfaction, previous to the commencement of hostilities (§ 51). But the treaty with the king of Syria had not yet received its consummation: and it concerned that monarch alone, without making any mention of his adherents. Therefore Manlius undertook the expedition against the Galatians, as a consequence or a remnant of the war with Antiochus, This is what he himself very well observed in his speech to the senate;4 and he even added, that his first measure was to try whether he could bring the Galatians to reasonable terms. Grotius more appositely quotes the example of Ulysses and his followers, — blaming them for having, without any declaration of war, attacked the Ciconians, who had sent succors to Priam during the siege of Troy.5


     1.    Book II. chap. xvii.
     2.    See Wolf, Jus Gentium. §§ 730 and 737.
     3.    De Jure Belli et Pacis, lib. iii. cap. iii. § 10.
     4.    Livy, lib. xxxviii.
     5.    Grotius, ubi supra, not. 3.