International law, body of laws assumed to be binding among nations by virtue of their general acceptance. The beginnings of international law lay in attempts to humanize the conduct of war. The seminal work of Hugo Grotius, On the Law of War and Peace (1625), was one such; he also formulated several important principles, including a legal basis for the sovereignty of states. The works of Grotius and his successors were widely acclaimed but never officially accepted; however, legal principles were increasingly incorporated into international agreements such as the Congress of Vienna as well as into the constitution of the United Nations. International laws may arise through multilateral or bilateral agreements, as with the Geneva Convention, or simply by long-established custom, as with a large part of maritime law. In some cases, as with the war crimes rulings of the Nuremberg trials, they may be said to arise retrospectively. Because few nations are willing to relinquish any sovereignty, the law lacks a true legislative body and an effective executive to enforce it. The International Court of Justice is the international judicial body, and the UN in the process of compiling an international legal code is the nearest thing to a legislature, but these bodies are limited by the willingness of states to accept their decisions. These difficulties have led some theorists to deny international law true legal status, but this is an extreme view. The need for international rules is widely recognized, as shown by the increasing tendency to anticipate problem areas such as space exploration and exploitation of seabed resources and to attempt to develop international rules to regulate them.