By Essex Law Research Team

In Prohibited Force: The Meaning of ‘Use of Force’ in International Law (Cambridge University Press, 2024), Dr. Erin Pobjie addresses the ambiguities surrounding the prohibition of ‘use of force’ under article 2(4) of the UN Charter, a foundational rule of international law designed to prevent war and maintain international peace and security. Article 2(4) prohibits States from using force against each other, except in cases of self-defence or UN Security Council authorisation, yet its interpretation is often unclear in complex, real-world situations. Recognizing these challenges, Dr Pobjie introduces her ‘type theory’ framework, which suggests that determining a prohibited use of force should involve a set of contextual requirements and a flexible set of ‘non-essential’ elements – including physical force, effects, gravity, and hostile or coercive intent – that are weighed together, rather than applied rigidly. With this framework, Pobjie brings analytical depth to ambiguous cases, refining our understanding of this cornerstone of international law. Adil Haque describes Prohibited Force as ‘an extraordinary book’ with a ‘striking and rare’ combination of theoretical sophistication and empirical rigour. 

Opinio Juris, a leading blog on international law, recently hosted a symposium to engage critically with Prohibited Force. The discussion opened with Dr Alonso Gurmendi’s introduction, followed by Professor Claus Kreß, who highlighted the book’s potential to strengthen the international legal order. Professor Adil Haque explored its implications for self-determination units, while Ambassador Tomohiro Mikanagi considered its relevance to cases of territorial acquisition. Professor Andrew Clapham underscored the framework’s real-world impact, noting that its insights could affect thousands of lives by shaping legal responses to blockades impacting food and humanitarian supplies in conflict zones like Yemen and Gaza. Professor James Green reflected on the strengths and potential limitations of type theory when applied to complex, borderline cases, and Professor Alejandro Chehtman highlighted the need to balance analytical sophistication with accessibility in practical settings. In her response, Dr Pobjie engaged with each contributor’s insights and critiques, underscoring her framework’s potential to foster richer discourse on the prohibition of force and its role in advancing international peace and security. 

The full symposium can be read here: Symposium on Erin Pobjie’s Prohibited Force: The Meaning of ‘Use of Force’ in International Law – Introduction – Opinio Juris  

Prohibited Force is available open access for all readers thanks to the University of Essex’s Open Access Fund: https://doi.org/10.1017/9781009022897