Relative to the abundance of literature devoted to the legal significance of UN authorisation, little has been written about whether the UN’s failure to sanction an intervention can ever make it immoral. This is the question that I take up here. I argue that UN authorisation (or lack thereof) can have some indirect bearing on the moral status of a humanitarian intervention. That is, it can affect whether an intervention satisfies other widely accepted justifying conditions, such as proportionality, “internal” legitimacy, (...) and likelihood of success. The more interesting question, however, is whether the UN’s failure to provide a mandate can make a humanitarian operation unjust independently of these other familiar considerations. Is a proportional, internally legitimate humanitarian intervention, with a just cause and strong prospect of success, still morally unacceptable if it is not approved by the United Nations Security Council? This is the question that I turn to in the second half of the paper. (shrink)