The Statute establishing the International Court of Justice specified the following sources of International Law: international conventions; international customs;……
the general principles of law recognised by civilised nations; and judicial decisions and teachings of the most highly qualified publicists of the various nations, as subsidiary means for determination of rules of law. For purposes of clarity we consider them in the following order with an addition of a few more.
Roman law:
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Roman law formed a complete and general code from which most of the Continental Countries in Europe derived their legal principles. It also provided a basis to cover the early relations of the States. The jus gentium, which was applied to the dealings of citizens belonging to different nations, was based upon justice, equity and commonsense.
Besides, the jus gentium emphasised the idea of moral obligations which were equally binding on all the States. Finally, the Roman idea of equality of all citizens before law led to the idea of the equality of sovereign States in International Law.
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Works of Eminent Writers:
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Works on history and biography provide useful information with respect to wars, diplomacy, treaties and alliances. But more important are the writings of eminent jurists who have reduced to a system the rules of conduct adopted by sovereign and independent States in their dealings with one another.
These writers, by showing what rules nations actually do observe, by interpreting general opinion on given questions, and by giving definitions and modifications of previous rules based on general consent, provide a source of international law.
The first and the most prominent among them was Hugo Grotius. His book, On the Law of War and Peace, exercised enormous influence on the external relations of all the States. Then follows a long list of able lawyers, like Byn Ker Shoek, Wolf, Vattel, Kent, Wheaton, Manning, Woolsey, Westlake, Lawrence and Hall who are usually recognised as authorities on International Law, and statesmen of all times have accepted their opinions as authorities.
It may be noted that the mere opinion of one individual writer has, by itself, no binding force. Nevertheless, the opinions of eminent jurists carry a great force and it is customary to cite them as authorities.
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Kent says, “In cases where the principal jurists agree, the presumption will be very great in favour of the solidity of their maxims; and no civilised nation that does not arrogantly set all ordinary law and justice at defiance will venture to disregard the uniform sense of the established writers on International Law.”
Treaties, Alliances and Conventions:
Treaties, conventions and alliances, either for commercial or political purposes, form an important source of International Law. Treaties are compacts entered into by States, wherein the contracting States agree to observe given rules of conduct in their intercourse with one another.
They are concluded cither in accordance with the existing rules of International Law, or some new rules mutually agreed upon, modifying the previous ones. When most of the States become parties to such a treaty, convention or alliance, they tend to create an International Law.
The most important treaties that form sources of International Law deal with questions of territory, like the treaties of Westphalia (1648), Utrecht (1713), Paris (1763), or the transfer of sovereign rights, as the treaties of Varsailles (1783), and Paris (1856), or conduct to be observed during war by both belligerent and neutral countries. The examples of the latter are the Geneva Convention (1864) and the Brussels Conference (1890).
Municipal Law:
In the Municipal Law of every State may be found germs of International Law. The Municipal Law of every country regulates the questions of citizenship and naturalisation, neutrality, tariffs, extradition, diplomatic and consular services, etc.
All these subjects are of international interest and the decisions on these questions are cited as precedents in international negotiations. Similarly, admiralty questions dealing with prize cases are based almost entirely on international usage. Some of the most important decisions of the Supreme Court of the United States relate to questions fundamentally international.
United Nations:
Perhaps nothing has a greater impact on the development of International Law than the evolution of the United Nations. Broadly speaking, the United Nations has three main functions concerning law: it is an originator, an applier, and a developer of International Law. Of course these functions overlap each other to a very real extent.
For example, it is often by ‘applying’ law that law is in fact developed. But these three threads are disentangled, for the relevance of law to the United Nations cannot be appreciated without having all these threads.
Decisions in International Cases:
It is customary with the States to refer their disputes to international tribunals or courts of arbitration or conferences for adjudication. These decisions are accepted as precedents and become part of International Law.
Sometimes the cases of dispute are referred to international conferences. During modem times, The Hague, Washington and Lausanne conferences have made valuable contributions to International Law. The member-States of the League of Nations were pledged not to go to war till the matter in dispute had first been submitted to arbitration.
The Covenant of the League of Nations provided the machinery by which peaceful settlements could be effected. The Council, the Assembly and the Permanent Court of International Justice all had jurisdiction to decide cases of dispute. The United Nations Charter also contains a similar provision.
The pronouncements of the Court are treated by nations as authoritative, and are taken as correct interpretation of the present state of International Law. Though the Court does not regard itself as bound by precedent, in fact it considerably relies on its own prior judgments and opinions.
Reason:
Another important source of International Law is reason. Cases often arise for which no precedent may exist, and for which there exists no established principle of tacit or explicit consent.
In such cases, those who administer International Law must resort to determining by reason the principles of equality to be applied.
By considering precedents, finding analogies, and expanding already accepted principles, they evolve a judicious principle to fit the cases under adjudication. Such decisions have come to constitute a body of precedents generally accepted as valid.
Customs:
International Law is essentially customary. Certain usages and practices which particular States adopted have been accepted and followed so generally that they have come to constitute a definite body of rules.
Many of the principles of conduct are based on analogies with principles regulating individual conduct. Here International Law resembles the English Common Law. It derives its authority from generally accepted principles of usage, and tests new cases by the application of precedents.
History of War and Diplomacy:
The history of wars, of negotiations and conclusion of treaties are fruitful sources for the development of International Law. The declarations on policy, like the Atlantic Charter and the Potsdam Agreement, and manifestoes issued from time to time also help the growth of International Law.
Opinions of Diplomats and Statesmen:
The correspondence between diplomats belonging to various States or between a government and its diplomatic agents accredited to other States is an important source of international usage.
Often such opinions are regarded as confidential, but the United States, Britain and other countries with democratic governments publish a greater part of their foreign correspondence.
Instructions issued by the States for the guidance of their Foreign Service representatives and to commanders of armed forces are also valuable in the growth of International Law.
The French Marine Ordinance of 1861 formed the basis of Prize law. The Instructions for the Guidance of the Armed Forces of the United States in the Field (1863) exercised a great influence in adopting more humane methods of warfare.