This chapter examines the changing place of the reciprocity principle within humanitarian law, as this legal field bears on America's conflict with Al Qaeda and kindred groups. The conclusion is that reciprocity occupies a more prominent place within such law, especially as it has been adopted by the United States and the U.K., than is generally acknowledged. This prominence has implications for assessing the legality of America's most controversial counterterrorism policies. It requires us to view these policies in a more favorable light than permitted by other accounts of the pertinent law, even those offered by the Bush administration itself. This is the case even if – for quite different reasons that are elaborated on in Chapters 10 through 12 – the United States should ultimately forswear some of these methods.

Let us begin by identifying various places where humanitarian law accepts and rejects reciprocity. I then ask whether the line between the two makes sense – logically or morally. The main contention here is that humanitarian law is incoherent in its treatment of reciprocation as a remedy for breach, yet is nonetheless correct in rejecting both the stark extremes of perfect reciprocity, on one hand, and its total repudiation, on the other. Given the complexity of combat's novel forms and the moral issues they present, it is inadvisable for the law to fix the boundary between these rival domains with any principled bright line. The incoherence of humanitarian law is not fatal to the enterprise, I conclude.