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International Law (often called Public International Law) is generally defined as that body of rules or laws that regulates the interactions or relations among nations. Generally speaking, it is nations (called "states" in international law) that are the subject of international law rather than individuals, corporations, or other entities; however, this statement is becoming less true.
Private International Law (called Conflict of Laws in the United States) can be distinguished from Public International Law in that the latter deals with the choice of laws to be applied when an event, transaction, or dispute (normally involving private individuals or entities) has connections with more than one jurisdiction. "Domestic" or "Municipal" law (to be distinguished from the more usual municipal law; i.e., city law, city ordinances) is that body of law applied within a particular nation as its own internal ("domestic", "municipal") law. When an American speaks of "Foreign" Law, s/he is speaking of the domestic or municipal law applied within its own territory by a foreign state, usually one outside our Anglo-American common law system. Lastly, Comparative Law is the study of differing legal systems or traditions in two or more nations of the world.
When you carry out research in American federal or state law, you look for primary sources like constitutions, statutes, regulations, or court opinions, and secondary sources like textbooks, periodical articles, and other textual material. Such is not the case with international law where one must locate (1) treaties, (2) rules of customary law, and (3) general principles of law accepted by civilized nations.
Treaties are agreements or contracts among nations, and the normal expectation is that treaties are to be performed in good faith. Nowadays, the vast majority of international law rules are found in treaties. In many cases, you will want to know if a treaty governs issues involved in your research and how it might be located. You will then read it to discover any treaty provisions that might be relevant. SEE: Any Pathfinders on treaties and treaty indexes.
The second source of international law is Customary Law. For a rule to become customary law, two elements must be present: first, there must be a practice, a course of action, or some kind of behavior followed by nations over a period of time; and secondly, that practice or course of action must be viewed by these nations as binding upon them. Originally (from about 1648 onward), international law grew mostly as new customary rules came into existence; but, over the last 100 years or so, treaty-made rules have become the far more dominant source for international law rules. There is really no complete, written compilation of international customary law, but one finds evidence of customs (and hence tends to proves their existence) in court decisions, writings of international law scholars, collections of diplomatic correspondence, and similar sources.
The third source for international law rules is "General Principles of Law Accepted by Civilized Nations". An example would be the rule of res judicata (that is, once you've had a fair hearing in court and have exhausted your appeals, the matter is closed and cannot be reopened). Whether you are in France, the United States, India, or Nigeria, there is some type of res judicata principle applied in the legal system. Thus, this type of rule would be a "general principle of law accepted by civilized nations". To locate materials on Customary International Law or on General Principles of Law, SEE: Any Pathfinders on the Online Catalog, Card Catalog, Textbooks, or Periodical Articles.
The rules stated by an American court in reaching its decisions are viewed thereafter as binding in one's own jurisdiction. This is not so in the international legal system where court opinions are viewed as persuasive or helpful in determining applicable rules. Thus, if one is trying a case outside the Anglo-American common law system, court opinions will be helpful as evidence of rules of law, but will not serve as binding precedent. SEE: Any Pathfinders dealing with the decisions of courts outside the U.S. In addition, problems of international law frequently come before American courts; thus, one may find international law cases through the usual means of access to U.S. case law. SEE: Any Pathfinders on Digests, ALRs, Shepards, Legal Encyclopedias.
Another secondary source for international law is the writings of scholars in the field. In fact, outside the Anglo-American legal tradition, jurists give greater weight to scholarly writings which, consequently, contribute greatly to the growth and development of the law. As continental European scholars played a large role in the development of international law, the greater deference to legal writers has carried over into international law generally. To locate scholarly writings on international law, SEE: Any Pathfinders on the Online Catalog, Card Catalog, Textbooks, Periodical Articles.
Lastly, various international organizations (examples: U.N. Security Council, U.N. General Assembly) may from time to time adopt resolutions that are framed in a legal format. Under U.S. practice, these resolutions per se are not a source of international law. But, these resolutions may become rules of international law if embodied in a treaty or by gradually evolving as a customary practice viewed as binding; that is, by becoming a rule of customary international law.