The diplomatic protection of citizens abroad is a comparatively modern phenomenon in the evolution of the state, in constitutional and in international law. Not until the legal position of the state toward individuals, both its own citizens and aliens, and of states between themselves, had become clearly denned in modern public law, did diplomatic protection become a factor in international intercourse. A discussion of the subject therefore involves a preliminary study of three distinct legal relations, first, between the state and its own citizen; secondly, between the state and aliens resident within it; and, lastly, the relations of states among themselves with respect to their rights over and their international responsibility for delinquencies toward aliens.