Both radical rebellion and humanitarian intervention aim to defend citizens against tyranny and human rights abuses at the hands of their government. The only difference is that rebellion is waged by the oppressed subjects themselves, while humanitarian intervention is carried out by foreigners on their behalf. In this paper, it is argued that the prudential constraints on war (last resort, probability of success, and proportionality) impose tighter restrictions on, or demand more of, humanitarian interveners than they do of rebels. Specifically, I argue that rebels enjoy exemptions from the success principle that do not apply to humanitarians, and that rebels are not constrained by the foreseen mediated consequences of their actions - consequences that are interceded by the agency of other parties. The same cannot be said for intervening states. If this is right, then it is possible for a humanitarian intervention to fall short of the prudential conditions of legitimate war despite being expected to accomplish no less, and to cost no more, than a rebellion which is rightly judged to satisfy these conditions.