Oxford University Press Blog: “Through the power of precedent, international incidents involving the use of force help to clarify the meaning and interpretation of jus ad bellum, the corpus of rules arising from international custom and the United Nations Charter that govern the use of force. UN Charter Article 2(4) forbids states from using force in their international relations. Exceptions to this prohibition are acts taken in self-defence under UN Charter Article 51 or under the auspices of a UN Security Council authorization to use force under Article 42. States can also consent that another state use force in its territory, for example to combat rebel or terrorist actors. In certain cases, state practice gives rise to new interpretations of existing rules or novel exceptions emerge. Through the study of precedents scholars often consider whether or not there has been a shift in the legal landscape. To give but a few illustrations, commentators have questioned if States take measures of self-defence under Article 51 to protect nationals abroad (a justification that has been invoked at various moments, for instance by Russia in the context of the crisis in Georgia in 2008), if a right to humanitarian intervention has emerged (a discussion triggered by the Kosovo crisis in 1999), or if self-defence under Article 51 can be invoked against non-state actors (a topical debate in the post 9/11 era). Consequently, depending on the precedent’s facts and the arguments invoked by the main protagonists different legal issues can be triggered…”