This book focuses on one of the most important issues of international law relating to the use of force. Using the premise that in the aftermath of the terrorist attacks of 9/11 the gap between the law and the state practice concerning use of force is increasingly widening the thesis argues that the law of self-defence stands on the brink of modification and change. As the collective security system finds itself to be relevant and effective in less and less crises the question arises as to whether the UN Charter rules on the use of force reflect contemporary international law or whether we have returned to the pre-Charter customary rules which would justify wider range of uses of force than the Charter rules. Chapter 1 examines the changing context of the law of self defence. Chapter 2 refers to the theoretical framework of self-defence before 9/11. Chapters 3 and 4 assess the legality of the US-led military operations in Afghanistan (2001) and Iraq (2003). The book also addresses how the Iraq crisis in 2003 shed light on all the flaws of the collective security system with the effect of even questioning the relevance of the Security Council. It assesses the impact of Security Council resolutions, passed after the event, on unilateral use of force. Chapter 5 examines the right of self-defence in the light of recent International Court of Justice jurisprudence. Chapter 6 argues that the contemporary paradigm of use of force does not address the modern security challenges and changes in technology and maintains that there is every need to reconceptualise the right of self-defence and allow for preemptive forms of self defence in cases of threats from terrorists and proliferation of weapons of mass destruction.