I have written this book with the belief that international law, since at least the sixteenth century, has clearly prescribed when a state can lawfully exercise its inherent right of self-defence. In this regard, the law’s substantive rules have simply re fl ected the human defensive instinct of striking fi rst in the face of an imminent threat of harm in order to avoid injury to self, or to others. International law, as does the criminal law on the municipal level, thereby ful fi ls its two fundamental purposes of distinguishing lawful from unlawful force and of protecting the threatened from attack while reducing the instance of force generally. Given the high stakes involved when a state is compelled to use force to defend itself, the necessity for such simplicity, in both understanding and application, is patent. I also believe that the substantive rules of international law have always possessed the intrinsic fl exibility to accommodate the continuous evolution of weapons and the emergence of new forms of aggressors and their tactics without impairing a state’s authority to defend itself from being attacked. Changing these rules or advocating for their wider scope is, therefore, unnecessary for achieving the law’s two fundamental purposes. My book resurrects the simplicity of the law, something which has been obscured since the 1960s amid technical argument and divergent state practice. Thank you, my dear wife Jacqueline, for your love, patience and sacri fi ce along the way.