This article scrutinizes the phrase ‘all necessary means to protect civilians under threat of attack’, contained in the United Nations (UN) Security Council Resolution 1973 (2011) authorizing military force in the Libyan Arab Jamahiriya (Libya). It assesses both the meaning of this phrase and the legal regime pursuant to the resolution. That regime challenges the teleological separation but concurrent application of the law on the use of force (the jus ad bellum), and the law applicable in international and non-international armed conflict [the jus in bello or international humanitarian law (IHL)]. Security Council Resolution 1973, and its understanding of the term ‘civilian’, should be read in accordance with other international law norms; prima facie conflict of the resolution with IHL on the issue of targeting can be resolved. The resolution was however ambiguous on when force can be used. It is suggested that Resolution 1973 required a demonstrable risk of indiscriminate attack to civilians, per se necessity and jus ad bellum proportionality, the latter exceeding IHL’s concept of proportionality because of the specificity of the resolution’s aim. In examining the concurrent application of the jus ad bellum and the jus in bello in the context of specific interventions in Libya, the criticism that some states contributing coalition forces overstretched their mandate is corroborated. A combination of the resolution’s ambiguity and political considerations lie at the heart of that overstretch. In developing international law for analogous situations, the intervention is likely to exacerbate existing quarrels over future council action to protect civilians.