During the Cold War, I was startled awake when the ground shook, frightened that nuclear war had begun; but it was Los Angeles, and only an earthquake. I have written this book with the wish that we all someday may awaken from the nightmare of armed conflict.

It was during the Vietnam War that I first learned about just war theory. In terms of just war principles, my belief was then, and still is, that the Vietnam War was unjust. After the Cold War, I was shocked by the genocide in Rwanda. In terms of just war principles, I believed then, and still do, that armed intervention there would have been just. My first article about just war theory, which featured the case of Rwanda – ‘Is armed humanitarian intervention to stop mass killing morally obligatory?’ (2001) – expressed theses that are precursors of theses in this book.

Truly, some uses of armed force are just, and some uses of armed force are unjust. The problematic of just war theorising is to formulate and support moral principles by means of which responsible agents can determine correctly whether a particular use of armed force would be just or unjust. This book is devoted to the study of such principles, which might be more aptly termed ‘unjust and just war principles’.

As the book's subtitle indicates, my purpose is to develop a just war theory that is cosmopolitan, whereas more traditional just war theories tend to be state-centric.