The moral challenges of using force against enemies who hide amongst innocent populations is as old as warfare itself, though reshaped somewhat by modern technology and militaries. This chapter explores this modern challenge from the perspective of the traditional ethical framework of Western society bequeathed to it by Christianity, namely the just war tradition. It argues that justice, in a broad sense, is required to defeat insurgencies, and this extends beyond the battle itself to the development of what St Augustine called tranquillitas ordinis, or a justly ordered society. Modern just war requires considerations beyond those present in the traditional just war theory structure; just policies for war. These policies include not only those that shape armies and guide their use, but also those for establishing a justly ordered society.

Classical just war theory operates from a ‘will to peace’ and is traditionally divided into the ethical considerations of just recourse to war (jus ad bellum) and just conduct in war (jus in bello). This chapter is primarily concerned with the latter and only then from the viewpoint of how to combat radical Islamic terrorism and insurgencies. The chapter proceeds by considering the ‘moral calculus’ of the ethical decisions in war, then briefly presents information specific to fighting terrorists and insurgents in the twenty-first century.