§ 21 International Law
International law or the law of nations consists of those rules which govern sovereign states in their relations and conduct towards each other. All men agree that such a body of law exists, and that states do in fact act in obedience to it; but when we come to inquire what is the essential nature and source of this law, we find in the writings of those who deal with it a very curious absence of definiteness and unanimity. The opinion which we shall here adopt as correct is that the law of nations is essentially a species of conventional law—that it has its source in international agreement—that it consists of the rules which sovereign states have agreed to observe in their dealings with each other.
This law has been defined by Lord Russell of Killowen68 as “the aggregate of the rules to which nations have agreed to conform in their conduct towards one another.” “The law of nations,” says Lord Chief Justice Coleridge,69 “is that collection of usages which civilised states have agreed to observe in their dealings with each other.” “The authorities seem to me,” says Lord Esher,70 “to make it clear that the consent of nations is requisite to make any proposition part of the law of nations.” “To be binding,” says Lord Cockburn,71 “the law must have received the assent of the nations who are to be bound by it. This assent may be express, as by treaty or the acknowledged concurrence of governments, or may be implied from established usage.”
The international agreement which thus makes international law is of two kinds, being either express or implied. Express agreement is contained in treaties and international conventions, such as the Declaration of Paris or the Convention of the Hague. Implied agreement is evidenced chiefly by the custom or practice of states. By observing certain rules of conduct in the past, states have impliedly agreed to abide by them in the future; by claiming the observance of such customs from other states, they have impliedly agreed to be bound by them themselves. International law derived from express agreement is called in a narrow sense the conventional law of nations, although in a wider sense the whole of that law is conventional; that part which is based on implied agreement is called the customary law of nations. The tendency of historical development is for the whole body of the law to be reduced to the first of these two forms—to be codified and expressed in the form of an international convention, to which all civilised states have given their express consent. Just as customary civil law tends to be absorbed in enacted law, so customary international law tends to be merged in treaty law.
International law is further divisible into two kinds, which may be distinguished as the common law of nations and the particular law of nations. The common law is that which prevails universally or at least generally among all civilised states, being based on their unanimous or general agreement, express or implied. The particular law is that which is in force solely between two or more states, by virtue of an agreement made between them alone, and derogating from the common law.
International law exists only between those states which have expressly or impliedly agreed to observe it. Those states (which now include all civilised communities and some which are as yet only imperfectly civilised) are said to constitute the family or society of nations—an international society governed by the law of nations, just as each national society is governed by its own civil law. New states are received into this society by mutual agreement, and thereby obtain the rights and become subject to the duties created and imposed by international law.
Writers are, however, as we have already indicated, far from being unanimous in their analysis of the essential nature of the law of nations, and the various competing theories may be classified as follows:—
(1) That the law of nations is, or at least includes, a branch of natural law, namely, the rules of natural justice as applicable to the relations of states inter se.
(2) That it is a kind of customary law, namely the rules actually observed by states in their relations to each other.
(3) That it is a kind of imperative law, namely the rules enforced upon states by international opinion or by the threat or fear of war.
(4) That it is a kind of conventional law, as already explained. Having accepted the last of these theories as correct, let us shortly consider the nature and claims of the three others.