This article investigates the legal consequences of treaty signature not followed by ratification, focusing on France’s 64-year-old signature of the 1961 Convention on the Reduction of Statelessness. France’s engagement with the 1961 Convention is unusual, raising multiple questions. One question relates to the customary status of Article 18(a) of the Vienna Convention on the Law of Treaties (VCLT). This provision requires States to refrain from acts defeating a treaty’s object and purpose prior to ratification. However, France is not a party to the VCLT, and in any event the VCLT post-dates the 1961 Convention and does not apply retroactively. Therefore Article 18 does not apply to France unless equivalent obligations exist in customary law. Another question relates to France’s international obligations rooted in customary international law on statelessness. These obligations include rules of customary law which are codified in or have crystallised as a result of the 1961 Convention. After outlining the customary status of Article 18(a) VCLT and of key provisions of the 1961 Convention, the analysis turns to the role of France in drafting the Convention, its reservations, legislative practice and judicial engagement, and persistent failure to ratify despite legislative reforms and parliamentary debate. The article concludes that during the period between signature and ratification, France remains bound by two distinct sets of obligations arising from customary international law: those triggered by its signature under the obligation of a customary character reflected in Article 18(a) VCLT and those triggered by the customary obligations of States in relation to statelessness. Furthermore, the mere passage of time does not extinguish those obligations.

The International Law Commission’s (ILC) Draft Conclusions on Identification of Customary International Law represent an important attempt to provide organized doctrinal guidance on customary law-making. However, a conceptual issue figures prominently: the formation and development of custom are characterized in the Conclusions as distinct from the identification of custom, and the ILC offers little explanation as to why this must be the case. In essence, this distinction assumes that the formation of custom is an objective, spontaneous, and chronological process beyond the control of individual lawyers. Although seemingly innocuous, this assumption is directly responsible for various doctrinal problems, such as the relationship between the two traditional elements of custom, the requisite duration of its formation, and the ‘post-identification’ interpretation of custom. Drawing primarily on insights from historiography, this article explains why the formation and identification of custom should be understood as a single process, and why conceiving of it as a matter of narrative construction provides a better solution to the above-mentioned questions.

Recent conflicts have revived interest in a subject that has remained largely untouched since the trials of war criminals after the Second World War: prosecuting individuals for the crime of aggression. Due to its restricted jurisdictional regime, the International Criminal Court has so far been unable to investigate this crime and remains barred from doing so in, for example, the context of the Ukraine situation. When exploring alternative domestic (or hybrid) venues for prosecuting aggression, discussions have come to focus on the possibility of exercising universal jurisdiction (UJ), a concept increasingly used to counteract substantive selectivity of international criminal justice. However, scholars and practitioners have expressed differing views on the extent to which the notion of UJ has come to extend to the crime of aggression under customary international law. This study contributes to the ongoing debates by providing a legal-empirical analysis of the practice and opinio juris of 126 states, as presented to the Sixth Committee of the UN General Assembly under the agenda item on the ‘scope and application of universal jurisdiction’. These empirical quantitative and qualitative insights provide a foundation for evaluating the customary status of UJ over the crime of aggression and exploring the legal challenges of prosecuting aggression under this jurisdictional title.

This article considers the legal status of the prohibition on gender-based violence against women. The Convention on the Elimination of All Forms of Discrimination against Women (CEDAW) does not contain a prohibition on violence against women as such. However, the CEDAW Committee found in its General Recommendation 19 that the definition of discrimination against women in Article 1 CEDAW includes gender-based violence against women. As general recommendations are not binding, it is often suggested that, aside from the regional treaties on the issue, the prohibition exists only under soft law. By contrast, this article argues that the prohibition exists also under hard law. Whether the definition of discrimination against women in Article 1 CEDAW includes gender-based violence against women is a question of treaty interpretation.

The article contends that, in line with Article 31(3)(b) of the Vienna Convention on the Law of Treaties, the subsequent practice of States Parties in the application of CEDAW has established their agreement that discrimination against women in Article 1 includes gender-based violence against women. It argues further that General Recommendation 19 served as the catalyst for this subsequent State practice. The article also explores the relationship between treaties and customary international law and contends that CEDAW has generated rules of customary international law, namely that gender-based violence against women is prohibited and that there is an obligation on States to exercise due diligence to eliminate it. The large number of States Parties to CEDAW has not proved an obstacle to the generation of these customary rules.

The 2025 International Court of Justice Advisory Opinion on Obligations of States in respect of Climate Change constitutes a diligent and reasoned exposition of the main bodies of international law on climate change. The Court read this law harmoniously and clearly identified States’ obligations as well as the legal consequences of breach. Under both treaty and customary international law, States must act with due diligence and do their utmost to mitigate climate change, including through action on fossil fuel production and consumption. The Court confirmed that the law of State responsibility applies if States fail to fulfil their obligations, and sketches a pathway for establishing causation of harm that would become relevant if reparation were sought.

This article examines the key dimensions of the Advisory Opinion, including the applicable law identified by the Court and its analysis of State responsibility, informing readers of the Court’s main findings and their consequences. It also puts forward a few reflections, including on the Advisory Opinion’s emphasis on international cooperation and finance flows, the Court’s views on sea-level rise and self-determination, the role of science in the Advisory Opinion and what the Court left for the future.

In the 2023 judgment of Nicaragua v. Colombia, the International Court of Justice ruled that, under customary international law, a State’s entitlement to a continental shelf beyond 200 nautical miles from its baselines is not permitted to extend within 200 nautical miles from the baselines of another State. In identifying this customary rule, the Court did not apply the two-element approach. The state practice relied upon by the Court to identify the general practice is not sufficiently widespread, representative, or consistent. The opinio juris is inferred from such state practice, which is not necessarily driven by a sense of legal obligation. The Court’s assertion of the customary rule constitutes, in effect, a rewriting of the relevant provisions of the United Nations Convention on the Law of the Sea, amounting to a legislative exercise.

This article examines the role of customary international law in regulating SRM by analysing competing interpretations of the customary law principle of prevention and their implications for SRM governance. Existing customary law limitations are largely overlooked in current policy and expert discussions around the future of SRM, which seemingly proceed on the basis that there are no universally applicable limitations for states to develop SRM technologies should they decide so. The paper contrasts this view and argues, from a positivist point of view, that the customary principle of prevention does pose certain limits for states even before SRM-caused transboundary environmental harm occurs. It distinguishes between a retrospective and future-oriented dimension of the prevention principle, and it depicts three scenarios for how the prevention obligation may limit the development and deployment of SRM technology depending on how States (and international legal advisors) conceptualise the temporal scope and normative content of the prevention principle. The article also examines the implications of the different configurations of the customary law obligation for an eventual SRM treaty.

In the wake of Russia’s aggression against Ukraine, international lawyers and policy advisors are considering the tools that are available to third States that wish to respond to the serious breach of international law and support Ukraine. Within this context, the question of third-party countermeasures is once again highly relevant. Though the topic was contentious during the drafting of the Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA), doctrine on third-party countermeasures often argues that they are permissible under customary international law, even while acknowledging that opinio juris is lacking. Whereas it has been argued that this subjective requirement can be inferred, this article maintains that, given the ambiguity surrounding unilateral sanctions practice, it is necessary to demonstrate that States believe that they are legally permitted to adopt wrongful sanctions in response to a prior breach of an obligation erga omnes (partes). It is argued that the International Law Commission was right to not include third-party countermeasures in the final ARSIWA and that, while sanctions practice has seemingly flourished over the years, there has been little progress in conclusively establishing that third-party countermeasures are accepted as custom, as illustrated by the discussion on the confiscation of Russian State assets.

States have the sovereign right to pursue their social and economic development, often involving urbanization, industrialization, infrastructure development, and tourism promotion. Regrettably, these endeavors frequently lead to destruction or damage to the authenticity and integrity of their cultural heritage. In light of this, one might wonder whether and to what extent, under customary international law, states’ right to development must be harmonized with the need to preserve their own cultural heritage for the benefit of future generations. The present contribution examines various elements of international practice, including treaty practice, resolutions of international organizations, national legislation, and states’ conduct in actual cases to determine the existence of a customary prohibition or clear limits on damaging cultural heritage for social and economic development. It also reflects more broadly on the international regime for protecting cultural heritage during peacetime and the ambiguous relationship between the World Heritage Convention and customary law.