Just and Unjust Wars, by Michael Walzer, is in its third edition. This is a seminal work of brilliance and one of the most illuminating book I have read in a very long time. if you are interested in foreign policy, I highly recommend this book.
And though the book was written 25 years ago, it still has much bearing today (especially since the US has become so interventionist). this book will really make you think about Iraq.
This book tries to describe:
(1) when is war justified
(2) how the conduct of a war can be just or unjust