International law has been defined as consisting of ‘rules and principles of general application dealing with the conduct of states and of international organizations and with their relations inter se, as well as with some of their relations with persons, whether natural or juridical’. As underlined by this definition, international law is no longer exclusively limited to the relations between States. The twentieth century witnessed a dramatic evolution of the international legal system, characterised by two major developments. First, a quantitative change has taken place through the unprecedented growth of international rules, which now cover virtually all human activities. Second, this metamorphosis has been accompanied by a qualitative change, with the emergence of new subjects of international law, such as individuals, international organisations and multinational corporations.

However, States remain the primary subjects of international law. In a sensitive area such as international migration, it has become conventional to underline the centrality of sovereignty. According to the famous statement of the Supreme Court of the United States in 1892, ‘[i]t is an accepted maxim of international law, that every sovereign nation has the power, as inherent in sovereignty, and essential to its self-preservation, to forbid the entrance of foreigners within its dominions, or to admit them only in such cases and upon such conditions as it may see fit to prescribe’.