Our world is currently divided into territorial states that resist all attempts to change their borders. But what entitles a state, or the people it represents, to assume monopoly control over a particular piece of the Earth's surface? Why are they allowed to prevent others from entering? What if two or more states, or two or more groups of people, claim the same piece of land?

Political philosophy, which has had a great deal to say about the relationship between state and citizen, has largely ignored these questions about territory. This book provides answers. It justifies the idea of...