International law is the set of rules that govern relations between states, both individually and collectively. It consists of legal norms, principles, and practices and includes a wide variety of subjects and actors, including sovereign states but also international organizations and some individuals.

Classically, international law was defined by Bentham as a “system of laws governing the relations between states”. However, in modern times this original definition has been overtaken by an increasing number of subjects and actors and has evolved to incorporate more sophisticated structures and processes.

The sources of international law include international treaties, customary law and judicial decisions. These sources of law can be distinguished from general principles which are common to systems of national law.

These general principles are often shifting and too vague to form the basis of a coherent system of international law. They can also be mistaken for state practice and opinio juris which can make it easier to ignore them and the legal consequences of state behavior.

In this regard, international law resembles the political protection rackets of the 19th century, and it is at times more reminiscent of a war of attrition than a legal system. This is particularly true during periods when the United Nations Security Council is locked in a stalemate, such as during the Cold War.

The most important question for international law is whether it can achieve legitimacy on the grounds that its rules can be interpreted, imposed and enforced, in the same way as domestic laws. This involves five criteria: a centralized legislative authority, courts with compulsory jurisdiction, effective enforcement mechanisms, forms of international solidarity necessary to make democratic governance feasible and a sense that the process of constructing international law is fair and open to all (Crawford 1979; Lefkowitz 2020).