International Law and the International Court of Justice

International law traces much of its modern doctrinal foundation to Hugo Grotius, the Dutch jurist whose De Jure Belli ac Pacis (On the Law of War and Peace, 1625) is widely credited with systematizing principles of natural law governing relations between sovereign states, though customary practices such as diplomatic immunity and the laws of war have far older roots. Article 38 of the Statute of the International Court of Justice identifies the modern sources of international law as international treaties and conventions, international custom evidenced by general state practice accepted as law, general principles of law recognized by civilized nations, and, as subsidiary means, judicial decisions and the teachings of highly qualified publicists. The 1969 Vienna Convention on the Law of Treaties, itself now largely regarded as codifying customary international law even for states that have not ratified it, sets out the rules governing how treaties are formed, interpreted, and terminated, and enshrines the principle pacta sunt servanda — that treaties are binding on the parties and must be performed in good faith.