In public international law, the question of supremacy between international obligations and national law arises frequently. This essay examines the issue by drawing on realist and institutionalist perspectives, alongside doctrinal analysis from Akehurst. It argues that international law formally prevails on the international plane, yet national law often governs domestically in dualist systems, with conditions shaped by incorporation, state interest, and institutional constraints.
Theoretical perspectives on hierarchy
Realist analysis, exemplified by Mearsheimer (1994), contends that states prioritise national interests over international commitments. Institutions are viewed as epiphenomenal; when conflict emerges, powerful states follow domestic imperatives because no effective enforcement mechanism exists. Consequently, national law governs in practice whenever vital interests are at stake. By contrast, Keohane and Martin (1995) maintain that institutions facilitate cooperation by reducing transaction costs and providing information. They argue that once states embed obligations domestically, reputational and linkage pressures make defection costly, suggesting that international rules can prevail under conditions of dense institutionalisation and repeated interaction.
Doctrinal position in the United Kingdom
Chapter 4 of Akehurst’s modern introduction to international law clarifies that the United Kingdom adopts a dualist approach. Treaties require legislative incorporation to produce domestic legal effect; customary rules may be received only insofar as they do not conflict with statute. In cases such as Maclaine Watson v Department of Trade, courts have confirmed that unincorporated treaty provisions do not override national law. On the international level, however, the United Kingdom remains bound by its treaty obligations and may incur state responsibility for non-performance. Thus, national law governs internally until Parliament acts, while international law governs externally regardless of domestic implementation.
Conditions determining supremacy
Three conditions shape outcomes. First, the constitutional tradition of the state determines automatic reception or the need for incorporation. Second, the density of institutional oversight, as Keohane and Martin emphasise, raises the cost of non-compliance. Third, relative power, highlighted by Mearsheimer, allows stronger states to resist international pressure more effectively than weaker ones. In the absence of incorporation or strong institutional sanctions, national law typically prevails in domestic courts; yet the state remains internationally accountable.
In conclusion, no universal hierarchy exists. International law governs relations between states, whereas national law generally governs within the domestic legal order of dualist jurisdictions. The balance ultimately depends upon incorporation requirements, institutional embedding, and the distribution of power among states.
