The invasion of Iraq by the United States, United Kingdom, and Australia in 2003 prompted an enormous volume of debate around the world as to whether it was the right thing to do and whether or not it was legal in terms of the Charter of the United Nations. Many who had previously had little interest in international law were now able to conduct a discussion as to whether the invasion had or had not been authorized by the United Nations Security Council. Iraq thereby brought starkly into focus the body of international law that seeks to govern whether and when countries may resort to the use of force. In the contemporary world, this body of law has as its centerpiece the Charter of the United Nations of 1945. In the scheme of world history it is therefore a relatively recent development. The attempt to place strict limits on the occasions when a country can use force was one of the great governance innovations of the twentieth century. Although political philosophers and lawyers had long called for this development, there was no proof that it would work. It was a bold experiment, and one that the world could not afford to see fail. This book responds to this heightened interest in international law addressing the use of force and traces the story of this great experiment in world politics.