Speaker: Dr Penelope Ridings, International Law Commission

Lecture summary: In the last several decades, scholarly views of due diligence in international law have shifted from due diligence as a primary obligation under customary international law, to due diligence as a standard of conduct attached to a primary obligation. Thus, for example, due diligence is required to meet a State’s obligation of protection (of the environment) or of prevention (of genocide). The International Court of Justice in its Advisory Opinion on Climate Change adopted such an articulation and stated that due diligence is a standard of conduct and States have a duty to prevent significant harm to the environment by acting with due diligence. The Court not only reinforced the importance of the customary international law obligation not to cause significant harm to the environment but placed this within the ‘no harm’ principle, as expressed in the Corfu Channel case. However, the Court did not expressly articulate whether there was a broader obligation of due diligence that applies not only to the prevention of environmental harm, but also to the prevention of other harms to the rights and interests of States. Due diligence is thus as a crossroads. Has the ICJ essentially sought to bridge the gap between on the one hand the notion of due diligence as an obligation on a State not to permit activities subject to its jurisdiction or control which causes harm to the rights and interests of other States, and on the other hand the notion of due diligence as a standard of conduct attached to a primary obligation? Has the Court opened the door to finding a general customary international law obligation not to cause harm to the rights and interests of other States? Or has it confined due diligence to its status as a standard of conduct attached to a primary obligation? This lecture will discuss this pivotal point which is central to the elucidation of the foundation and scope of the due diligence obligation under international law.

Dr Penelope Ridings is a Member of the International Law Commission and New Zealand Barrister practising in the field of public international law. In 2025 she was appointed the ILC Special Rapporteur for the topic ‘Due Diligence in International Law’. She was formerly New Zealand’s Chief International Legal Adviser in the Ministry of Foreign Affairs and Trade and a New Zealand diplomat. She was Agent for New Zealand before the International Court of Justice in Whaling in the Antarctic: Australia v Japan, New Zealand Intervening and before the International Tribunal on the Law of the Sea in the Request for an Advisory Opinion submitted by the Sub-Regional Fisheries Commission.

After moving to the New Zealand Bar, she has advised governments and international organisations on public international law including law of the sea, fisheries, environmental law, trade and investment, international security and international dispute settlement. She was Chair of the 2025 arbitration under the EU-UK Trade and Cooperation Agreement (UK-Sandeel) and Chair of the WTO appeal arbitration China – Enforcement of Intellectual Property Rights under the Multi-Party Interim Appeal Arbitration Arrangement. She has served on several ICSID ad hoc Annulment Committees, including as Chair, and as an independent panellist in disputes before the WTO. She has lectured in international law and contributed to several books and written articles on various aspects of international law.

This lecture was delivered on 13 February 2026 and is part of the Friday Lunchtime Lecture series.

Speakers: Prof Mads Andenas & Prof Johann Ruben Leiss, University of Oslo

Lecture summary: The lecture explores the systemic function of general principles in international law in light of the ongoing work of the ILC on general principles of law and recent practice of international courts and tribunals, such as the Climate Change Advisory Opinion of the International Court of Justice from 2025. In its first part, the lecture examines the ILC’s approach to the systemic function of general principles and comments of states on the ILC’s work. In its second and third part, the lecture discusses the two main features of the systemic function of general principles, namely their contribution to inter-norm and inter-systemic coherence in international law. All general principles potentially fulfil a systemic function by their gap-filling role and inter-systemic communication through Article 38(1)(c) ICJ Statute. Several general principles have a systemic pull in inter-norm contexts as interpretative guidelines and inter-norm harmonisers and coordinators. In the relationship between different (sub)orders of international law (including European law and national legal orders applying international law), several principles provide for ‘hinge’ mechanisms and inter-system harmonisers which open legal (sub)orders to one another, and integrate them into (relative) unity, while others serve as inter-system coordinators or mechanisms for conditional closure of legal orders. This means, all general principles have a systemic function, whereas certain principles have more direct systemic function by virtue of their normative content. Through their systemic function, general principles contribute as a central cohesive force furthering international law’s character as a legal and (relative) unitary system. This system is characterized by a complex and dynamic interplay between a plurality of legal norms, orders, and sub-orders, including national legal orders, through systemic principles of openness, coordination, and conditional closure.

Chair: Prof Campbell McLachlan

This lecture was given on 6 February 2026 and is part of the Friday Lunchtime Lecture series at the Lauterpacht Centre.

Lecture summary: Empire is a big theme in international law. At the same time, the historical discussion on imperialism and international law had focussed primarily on the West European Empires. This presentation examines Russian and Soviet historical engagements with international law through imperial ideas and practices. Of the doctrines of international law, the ideas of state identity (continuity) and also termination of treaties via the doctrine of clausula rebus sic stantibus are examined, and how their use has served the imperial construction and practice of international law in Russia. Understanding the history of international law in Russia through the lens of Empire helps us inter alia to situate Russia's war against Ukraine.

Lauri Mälksoo is Professor of International Law at the University of Tartu in Estonia. He is member of the Institut de Droit International, of the Venice Commission of the Council of Europe and of the Estonian Academy of Sciences. He has published two monographs on the history and theory of international law in Russia and the Soviet Union at the Oxford University Press.

Chair: Prof Marc Weller

This lecture was given on 30 January 2026 and is part of the Friday Lunchtime Lecture series at the Lauterpacht Centre.