Contents: Part I. The Legal Nature of War: 1. What is war?; 2. The course of war; Part II. The Illegality of War: 3. An historical perspective of the legal status of war; 4. The contemporary prohibition of the use of inter-state force; 5. The criminality of war of aggression; 6. Controversial consequences of the change in the legal status of war; Part III. Exceptions to the Prohibition of the Use of Inter-State Force: 7. The concept of self-defence; 8. The modality of individual self-defence; 9. Collective self-defence; 10. Collective security; Conclusion.