Newly revised, this textbook provides an authoritative conceptual and practical overview of international law governing the resort to force.Following an introductory chapter, with a section on the key issues in identifying the law and actual and potential changes to it, the book addresses the breadth and scope of the prohibition of the threat or use of force and the meaning of 'force' as the focus of this.The book proceeds to address the use of force through the United Nations and regional organisations, the use of force in peacekeeping operations, the right of self-defence and the customary limitations upon this right, the controversial right of humanitarian intervention, and forcible interventions in civil conflicts.Updated to include greater focus on aspects such as cyber operations, the threat of force, and the 'human element' to the use force, as well as the inclusion of recent developments such as the 2022 Russian invasion of Ukraine, it seeks to address the contemporary legal framework through the prism of contemporary challenges that it currently faces.