The problems posed by what are called weapons of mass destruction (WMD) loom large in contemporary international politics. Their alleged presence in Iraq was the principal public reason for the Iraq war, and their actual absence an embarrassment, if not a political and moral disaster, for the invaders and their supporters. Iran's alleged pursuit of them is the focus of another international crisis, so clarification of the nature of such weapons and the distinctive dangers they pose, or even are believed to pose, calls for specific treatment here, even though some of the problems raised by WMD will require revisiting some matters that have been dealt with in earlier chapters.

Since there is usually a tremendous amount of destruction in war, whatever weapons are used, it may seem that the concentration on specific weapons as “weapons of mass destruction” is peculiar. Is the machine gun a weapon of mass destruction because its use has enabled the efficient and rapid killing and injuring of vastly more people than previous weapons? Is the aeroplane, especially the bomber? These are not normally viewed with the disapproval reserved to WMD. Why not? There seem to be two reasons. First, these weapons are not in themselves geared to the idea of mass destruction, though that has indeed proved to be a common employment.