International law is a system of rules and principles that exists outside the legal orders of individual states. It provides normative guidance and methods, and a common conceptual language to international actors-primarily sovereign states but increasingly non-state international organizations, corporations and individuals.

Most international law is not enforceable by force; its enforcement generally takes the form of diplomatic or economic sanctions. Nevertheless, breaching the rules of international law can reflect poorly on a state’s reputation and may make future relations more difficult.

Traditionally, there are three main sources of international law: treaties, customary international law and general principles of law. Article 38 of the Statute of the International Court of Justice defines customary international law as “the evidence of a general practice accepted as law.” This includes both State practice and the subjective conviction of States that certain conduct is required by international law, known as opinio juris.

A fourth source of international law is general principles of law that are common to systems of national law. International lawyers are often called upon to interpret and apply these principles to specific situations.

Despite the efforts of international organizations like the League of Nations and the International Court of Justice, the world descended into two calamitous world wars in the first half of the twentieth century, dealing a serious blow to the idealistic assumption that international institutions could tame human nature. Nevertheless, the United Nations and its treaty bodies continue to develop and codify international law in areas such as disarmament, human rights, the environment and trade.