Diplomatic protection, with its origin in the international arbitrations of the 18th and 19th centuries, is one of the oldest principles in customary international law which protects ‘nationals’ abroad by using state-to-state procedures as a corollary of sovereign power. From an orthodox positivist view, states have the right to utilise this institution as a corollary of personal jurisdiction. It may, however, conflict with the global scheme of human rights protection. Although the possible restrictions of a state’s discretion in its exercise of diplomatic protection have been discussed for a century, this question has not attracted the interest of practitioners until recently, when the detainees at Guantanamo Bay camp asked their governments to exercise diplomatic protection on their behalf. Against this backdrop, this paper examines how states exercise personal jurisdictional power with a special focus on the classifications of societal membership – national, citizen, and denizen – as a legal basis for invoking diplomatic protection in the international sphere. The first section focuses on the fragility of the concept of ‘nationality’ in international law. The second section illustrates a predominant trend for residence-oriented protection of individuals in international human rights law, and this link to residence is sometimes superior to their formal nationality or citizenship. Based on these arguments, the third section scrutinises possible constrains on the discretion of a state in its exercise of diplomatic protection.