What Are the Sources of International Law?
What Are the Sources of International Law?

Since there is no world government, there is no world Congress or parliament to make international law the way domestic legislatures create laws for one country. As such, there can be significant difficulty in establishing exactly what is international law. Various sources, however—principally treaties between states—are considered authoritative statements of international law. Treaties are the strongest and most binding type because they represent consensual agreements between the countries who sign them. At the same time, as stated in the statute of the International Court of Justice (ICJ), rules of international law can be found in customary state practice, general principles of law common to many countries, domestic judicial decisions, and the legal scholarship. 

Treaties. Treaties are similar to contracts between countries; promises between States are exchanged, finalized in writing, and signed. States may debate the interpretation or implementation of a treaty, but the written provisions of a treaty are binding. Treaties can address any number of fields, such as trade relations, such as the North American Free Trade Agreement, or control of nuclear weapons, such as the Nuclear Non-Proliferation Treaty. They can be either bilateral (between two countries) or multilateral (between many countries). They can have their own rules for enforcement, such as arbitration, or refer enforcement concerns to another agency, such as the International Court of Justice. The rules concerning how to decide disputes relating to treaties are even found in a treaty themselves—the Vienna Convention on the Law of Treaties (United Nations, 1969).

Custom. Customary international law (CIL) is more difficult to ascertain than the provisions of a written treaty. CIL is created by the actual actions of states (called “state practice”) when they demonstrate that those states believe that acting otherwise would be illegal. Even if the rule of CIL is not written down, it still binds states, requiring them to follow it (Dinstein, 2004).

For example, for thousands of years, countries have given protection to ambassadors. As far back as ancient Greece and Rome, ambassadors from another country were not harmed while on their diplomatic missions, even if they represented a country at war with the country they were located in. Throughout history, many countries have publicly stated that they believe that ambassadors should be given this protection. Therefore, today, if a country harmed an ambassador it would be violating customary international law.

Similarly, throughout modern history, states have acknowledged through their actions and their statements that intentionally killing civilians during wartime is illegal in international law. Determining CIL is difficult, however, because, unlike a treaty, it is not written down. Some rules are so widely practiced and acknowledged by many states to be law, that there is little doubt that CIL exists regarding them; but other rules are not as universally recognized and disputes exists about whether they are truly CIL or not.

General Principles of Law. The third source of international law is based on the theory of “natural law,” which argues that laws are a reflection of the instinctual belief that some acts are right while other acts are wrong. “The general principles of law recognized by civilized nations” are certain legal beliefs and practices that are common to all developed legal systems (United Nations, 1945).

For instance, most legal systems value “good faith,” that is, the concept that everyone intends to comply with agreements they make. Courts in many countries will examine whether the parties to a case acted in good faith, and take this issue into consideration when deciding a matter. The very fact that many different countries take good faith into consideration in their domestic judicial systems indicates that “good faith” may be considered a standard of international law. General principles are most useful as sources of law when no treaty or CIL has conclusively addressed an issue.

Judicial Decisions and Legal Scholarship. The last two sources of international law are considered “subsidiary means for the determination of rules of law.” While these sources are not by themselves international law, when coupled with evidence of international custom or general principles of law, they may help to prove the existence of a particular rule of international law.

Especially influential are judicial decisions, both of the International Court of Justice (ICJ) and of national courts. The ICJ, as the principal legal body of the United Nations, is considered an authoritative expounder of law, and when the national courts of many countries begin accepting a certain principle as legal justification, this may signal a developing acceptance of that principle on a wide basis such that it may be considered part of international law.

Legal scholarship, on the other hand, is not really authoritative in itself, but may describe rules of law that are widely followed around the world. Thus, articles and books by law professors can be consulted to find out what international law is.

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