Until the mid-19th century, international law was based on the idea of independent sovereign states. The European Renaissance mapped the structure of international law. Before the emergence of the UN Charter, however, the earliest examples of international law date from the ancient Middle East, where cooperative agreements were common. In Mesopotamia, for example, the Lagash and Umma treaty was signed in 2100 BCE. In Egypt, the pharaoh Ramses II and Hattusilis III concluded their treaty in 1258 BCE. In addition, various Middle Eastern empires negotiated treaties and pacts with each other.

Bentham’s classic definition of international law describes it as “rules that govern international relations between nations.” This definition, however, omits individuals and other international organizations. Today, international law is an evolving set of principles, rules, and practices that govern states and their citizens. Many of these principles are binding but not directly applicable. Because of this, a state may object to an international law rule, claiming that it is not applicable to its people.

While courts decide what a nation’s laws say, the application of international law varies widely between nations. In some cases, the courts apply international law to domestic matters, and in others, international law is based on custom and precedent. For example, a nation’s domestic laws may be unenforceable if it is based on customary law. Despite this, many nations still follow the international laws that apply to their countries. Hence, the enforcement of international law is not as uniform as domestic law.