International law defines the legal responsibilities of states in their relations with each other and with individuals within State boundaries. It covers a wide range of topics including human rights, refugees and migration, international crime, the treatment of prisoners and the use of force.

The history of international law is shaped by various factors, including the rise of Christian civilization, European exploration and navigation, the formation of centralized governments, the development of science, humanism and individual rights, and the increase in trade and wealth. This growth in international activity required a new legal framework to regulate the conduct of states and peoples across national boundaries.

Early international law derived from Christian and Greco-Roman sources. The 16th century philosopher Alberico Gentili, for instance, argued that positive law (jus voluntarium) was based on custom and treaties commonly consented to by various states.

In the 18th and 19th centuries, scholars began defining and developing international law. The positivism school of writers emphasized that international law should be derived from actual state practice rather than Christian or Greco-Roman sources.

This school of thought was opposed by the natural law approach, which argued that international norms should be based on axiomatic truths. These ideas influenced later developments in international law, especially in the 20th century.

A number of international organizations create binding law by issuing resolutions and opinions on issues such as human rights, environmental protection and the treatment of refugees. These can be enforceable by member states under Chapter VII of the United Nations Charter or by international tribunals (including courts and arbitration).