Some of the major sources of international law are as follows:
(1) Roman Law:
Roman law formed a complete and general code of dealings —the Jus gentium. Jus gentium was applied to the dealings of citizens belonging to different nations. From this general code most of the Continental Countries in Europe derived their legal principles.
Roman law provided a positive basis for International Law in two ways:
(a) By the idea of the law of nations;
(b) By contributing the notion of the equality of citizens before the law. This notion extended to the equality of sovereign states of International Law.
(2) Treaties and Alliances:
Treaties, Alliances, Conventions and Compacts either for commercial or for political purpose are an important source of International Law. They have helped a lot in the development of International Law. They define pre-existing rules or modify them. They may affirm existing rules or modify them.
The most important treaties which have helped a lot in the development of International Law are the Peace of Westphalia (1648), Treaty of Utrecht (1713), Treaty of Paris (1763 and 1856), Gevena Convention (1864), Brussels Conference (1890) and Treaty of Versailles (1783 and 1919). These treaties, alliances and conventions and conferences have played a vital role in the development of International Law.
(3) Customs and Conventions:
Customs and Conventions form another source of International Law. As a matter of fact, International Law is customary. The dealings of Ancient and Medieval States depended on customs and conventions. Certain usages and practices which were adopted by particular states became customs and conventions. These customs and conventions later on took the shape of a body of rules, generally accepted by all. The judgement of U.S. Supreme Court in the case of Packet Havana is worth-noting.
(4) Municipal Laws (State Laws):
Municipal Laws (State Laws) also form a source of International Law. The germs of International Law can be found in the Municipal Laws of a State. The statutes of Municipal Law of every State affect international relations.
Every State has some of its Municipal Laws which help a lot in the development of International Law. For example, the rules and regulations of Citizenship, Naturalisation, Nationalisation, Neutrality, and Extradition are some of the rules and regulations which have international bearings.
The rules and regulations affecting the exchange of ambassadors and envoys who represent one State in another State also have international bearings. These rules and regulations nave helped a lot in the development of International Laws.
(5) Works of Eminent Jurists:
Many eminent jurists have expressed their Views on International Law. Every International Court of Law attaches due importance to their views. Their views are taken into consideration before the declaration of the judgment and while cross-examining the international issues. For example, Hugo Grotius’s famous book, “On the Law of War and Peace” was published in 1625. Now this book is regarded as the Chief source of International Law. His book gave the theoretical foundation of International Law.
Likewise, in his famous book “Law of Nature and Nations” (1672) Pufendorf and in his “Diplomatic Code of the Law of Nations” (1693-1700) Zaibniz gave theoretical foundation of International Law. Bynker Shoek (I673-I743) who first dealt with navigation law.
Wolf (1679-1754) and Vattel (1714-1767) are the names of important writers of authority who have helped a lot in the development of International Law. Kent, Wheaton, Manning, Woolsey, Westlake, Lawrence and Hall are known as modern authorities on International Law.
(6) Decisions in International Cases:
Now it has become a custom with the states to refer to international courts, tribunal and conferences for adjudication. In modern times, the Hague and Washington Conferences have played a vital role in the development of International Law. The U.N. Charter also lays stress on the fact that the states engaged in disputes will not wage the war before arbitration. These days’ international disputes are resolved by the U.N. in a peaceful mannerer.
(7) History of War and Diplomacy:
History of war and diplomacy also forms a source of International Law. History of wars, of negotiations and conclusions of treaties as contained in protocols and manifestoes and all international transactions prove fruitful for the development of International Law. The declarations, made by several states during the war-period, like the Atlantic Charter and Potsdam Agreement also help a lot in the development of International Law.
(8) Opinions of Diplomats and Statesmen:
The written opinions of statesmen and lawyers contained in state papers and diplomatic correspondence in the Foreign Offices of States have helped much in the growth and development of International Law. These opinions are often confidential but in Democracy there is a greater tendency to publish them.
The important portions of these opinions are published in England and U.S.A. Despite this, state issue instructions for the guidance of their foreign service representatives and commanders of armed forces. These instructions also play a vital role in the growth and development of International Law.
(9) Decisions of League of Nations and United Nations Organisations:
League of Nations and United Nations Organisation have been very important international institutions of twentieth century. The U.N. lays stress on Direct Negotiations, Mediation, Arbitration and International Court of Justice at Hague. Decisions of International Court at Hague, and the resolutions passed by the U.N. have proved very fruitful in the growth and development of International Law.