The prudential constraints on the use of military force are those that prohibit resort to war when:

1. there are peaceful alternatives that have not been exhausted (the principle of last resort) and/or;

2. the foreseen costs of war are prohibitively high (the principle of proportionality) and/or;

3. war is unlikely to succeed in its objectives (the success principle).

It goes without saying that 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 impose higher costs, and so on – then intervention might be ruled out despite rebellion being a legitimate option. Various factors have been known to augment the costs associated with foreign intervention and to impede its success, a sample of which will be surveyed in this chapter. There is another more interesting possibility, however, which is that the prudential constraints impose tighter restrictions on, or demand more of, humanitarian interveners than they do of rebels. If this is right, then it is possible for a humanitarian intervention to fall short of these conditions despite costing no more, and accomplishing no less, than a rebellion which is rightly judged to satisfy them. This position will be considered at length in the following chapter.