This paper explores Subsidiarity as a constitutional principle in international law. A principle of subsidiarity regulates how to allocate or use authority within a political or legal order, and holds that the burden of argument lies with attempts to centralize authority. In EU law, a principle of subsidiarity is explicitly part of EU law at least since the Maastricht Treaty. Principles of subsidiarity are also found in the constitutions of many federal states. Some authors have appealed to a principle of subsidiarity in order to defend the legitimacy of several striking features of international law, such as the centrality of state consent, the leeway in assessing state compliance and weak sanctions in its absence. The article presents such defenses of state centric aspects of international/ law by appeals to subsidiarity, and find them wanting.

Different interpretations of subsidiarity have strikingly different institutional implications regarding the objectives of the polity, the domain and role of subunits, and the allocation of authority to apply the principle of subsidiarity itself. Five different interpretations are explored, drawn from Althusius, the US Federalists, Pope Leo XIII, and others. The choice among them has drastic implications for the appropriate authority of international law and institutions vs domestic authorities – and thus for what sorts of institutional or constitutional reconfiguration should be pursued. One upshot is that the Principle of Subsidiarity cannot provide normative legitimacy to the state centric aspects of current international law on its own. It stands in need of substantial interpretation, which must be guided by normative considerations. While some versions of subsidiarity may match current practices of public international law, these are more questionable than the accounts that grant states a less central role in a legitimate multi-level legal and political order. If subsidiarity is to serve as a ‘constitutional principle’ for public international law, many crucial aspects of our legal order must be reconsidered – in particular the standing and scope of state sovereignty.