This chapter initially examines philosophical approaches to the international use of force in an historical context before examining the development of the doctrine of collective security as the unifying value of international relations at the end of the First World War and subsequently. States’ right of self-defence is seen as an exception to this doctrine. Drawing on analytical legal theory and theories of legal reasoning, it explores the nature of an exception to a rule. This classification can be difficult to identify as legal propositions can compete rather than exist in a hierarchical rule-exception relationship. The parameters of self-defence as an exception to the doctrine of collective security and the prohibition on the use of force is explored in this light, casting doubt on the validity of contemporary attempts to expand self-defence to justify extra-territorial attacks on non-state actors within states deemed unwilling or unable to curb their hostile activity.
Key words - use of force, self-defence, collective security, just war theory, history of international law, sources of international law, institutional theories of international law, analytical legal theory, legal reasoning, UN Charter interpretation