When is war justified? When is intervention in the domestic affairs of other states justified? Is international terrorist activity ever justified? Is the policy of nuclear deterrence morally defensible? These questions are clearly of the utmost importance, yet scholars in the discipline of international relations have for the most part avoided moral theory. Part One of this book examines the reasons put forward (or implicitly accepted) for ignoring moral theory and finds that none of them stands up to close scrutiny. In Part Two a start is made towards the construction of a normative theory which will be useful in seeking answers to the crucial questions listed above. The theory put forward is called constitutive theory and it is argued that constitutive theory is more satisfactory than utilitarianism, order-based theories or rights-based theories. In the final chapter constitutive theory is applied to the moral problems surrounding international terrorism.