In "Killing in War", McMahans main concern is with the moral equality of combatants. The moral equality of combatants refer to the hypothesis that soldiers are equally liable to being attacked and killed, no matter whether they are fighting on a just or an unjust side of the war. McMahan argues that the traditional assumption of the moral equality of combatants, of which for example Michael Walzer in "Just and Unjust Wars" is a major proponent, is false, and that this has considerable implication for the moral status of the acts committed on either side in any war with both just and unjust sides.
McMahans ideas are highly original and for the most part clearly stated. The book is divided into five chapters, where the second chapter constitutes the major part of McMahans arguments: he puts forth a good deal of different arguments for the moral equality of combatants and dismisses them all. McMahans arguments are very intelligent, original and convincing, and he generally is very objective in his thought and does not let himself be decieved by conventional thinking. It is this willingness to devy convention which is the major force of the book.
Apart from the moral equality of combatants, McMahan also discusses possible excuses for unjustified behavior, mostly excuses made from combatants on the unjust side of a war. He also considers civilian immunity at length, and gives some very interesting arguments for why absolute civilian immunity is unfounded.
A weakness of the book is that it at times is very dense and academic. Particularly in the fourth chapter on the moral status of different types of threatening combatants, the text is difficult reading, perhaps unnecessarily so. McMahan spends a good deal of pages on categorizing various types of threats, and later applies them to the moral status of combatants on the unjust side of a war. In doing so, he develops a considerable amount of definitions ("Culpable threats", "Partially Excused Threats", "Innocent Threats", "Nonresponsible Threats"...), and it is rather difficult to keep track of the different categories. While McMahans intentions are obviously good, in the sense that he strives for a high quality of scholarship and argumentation, the unfortunate side effect is that the book becomes very dense at such a point. In general, the writing of McMahan is not as clear as that of, for example, Michael Walzer in "Just and Unjust Wars".
Nonetheless, the quality and originality of McMahans arguments set forth in his book are so great that the book must clearly be recommended as a very important, if at times somewhat dry and academic, piece of work on Just War Theory.
I bought this book as a core text for my philosophy dissertation on war ethics. Contrary to the standard model of warfare, famously publicised by Michael Walzer, Jeff McMahan provides us with an alternative account: a model of warfare that shifts the moral responsibility of conduct in, and leading to, war to include the individual. Military action as a response to political problems is coming under increasing scrutiny and the ethics of such action are becoming of increasing importance. Although I do not necessarily agree with all of McMahan's views presented here, I would highly recommend that anyone interested in political philosophy and the military read a copy of this book.