If a humanitarian intervention is expected to produce less favourable consequences than a rebellion in the same circumstances – to accomplish less than rebellion would, to cause more death and destruction – then the intervention might obviously be ruled out by the prudential constraints on war despite rebellion being permissible. Various factors have been known to augment the costs associated with foreign intervention and to impede its success, some of which were surveyed in the previous chapter. But up to this point I have simply assumed that the prudential principles are just as demanding of rebels as they are of humanitarians; that they hold all belligerents up to the same standards. I now want to call this into question and consider a more interesting possibility.

In the first half of the chapter I show that, intuitively, we hold humanitarian interveners to more stringent prudential standards than those to which we hold rebels. In other words, there is an asymmetry in the way that the prudential constraints are applied which has strong intuitive appeal. I then turn to explore whether there is any principled justification for this asymmetry. Our intuitions cannot be treated as sacrosanct. This is especially important since holding rebels and humanitarians to unequal or asymmetric prudential standards seems, on the face of it, to conflict with a basic principle of moral reasoning: ceteris paribus, other-defence is supposed to be permissible wherever self-defence is permissible. Call this the Defence Axiom.