The Responsibility to Protect (RtoP) played an important role in shaping the world's response to actual and threatened atrocities in Libya. Not least, the adoption of Resolution 1973 by the UN Security Council on May 17, 2011, approving a no-fly zone over Libya and calling for "all necessary measures" to protect civilians, reflected a change in the Council's attitude toward the use of force for human protection purposes; and the role played by the UN's new Joint Office on the Prevention of Genocide and the Responsibility to Protect points toward the potential for this new capacity to identify threats of mass atrocities and to focus the UN's attention on preventing them. Given the reluctance of both the Security Council and the wider UN membership even to discuss RtoP in the years immediately following the 2005 World Summit--the High-level Plenary Meeting of the 60th Session of the General Assembly that gave birth to RtoP--these two facts suggest that significant progress has been made thanks to the astute stewardship of UN Secretary-General Ban Ki-moon, who is personally committed to the principle. Where it was once a term of art employed by a handful of likeminded countries, activists, and scholars, but regarded with suspicion by much of the rest of the world, RtoP has become a commonly accepted frame of reference for preventing and responding to mass atrocities. Resolution 1973 is especially important because it is the first time that the Security Council has authorized the use of military force for human protection purposes against the wishes of a functioning state. As Paul Williams argued, the closest the Council came to doing so in the past was in Resolutions 794 (1992) and 929 (1994). In Resolution 794, the Council authorized the Unified Task Force to enter Somalia to ease the humanitarian crisis, but this was in the absence of a central government rather than against one--a point specifically made at the time by several Council members. (1) Similarly, in Resolution 929 the Security Council authorized the French-led Operation Turquoise, ostensibly to protect victims of the ongoing genocide in Rwanda. Operation Turquoise enjoyed the consent of the interim government in Rwanda as well as its armed forces. More recently, in Haiti, the Democratic Republic of Congo (DRC), Sudan, and Cote d'Ivoire, the Security Council has authorized the use of "all necessary measures" to protect civilians, but the peace operations in these countries all operate with the consent of the host state. (2) Having twice stated its readiness in Resolutions 1674 (2006) and 1894 (2009) to take "timely and decisive action" to prevent or halt mass atrocities, the Council has now set a precedent that it will not be inhibited as a matter of principle from authorizing enforcement for protection purposes without host state consent.