The United Nations was founded when the gravest danger facing mankind was aggressive war by industrialized states marching armies across borders. This landmark reconsideration of the UN by a panel of world leaders argues that its principles, strategies, and organizations must now be adapted to meet new security threats arising from poverty, disease, environmental degradation, terrorism, and the spread of weapons of mass destruction. The report's appendix offers 101 specific recommendations for reform (including expanding Security Council membership), making the case that a holistic, preventive approach to threats demands new commitments to development, human rights, state building, and peacekeeping. But its most important contribution will be to stimulate international debate about threats and the use of force. It seeks to foster a new consensus on a set of principles ("seriousness of purpose," "last resort") to guide the Security Council -- but here the report offers little more than a prayer and a hope, as these principles only reaffirm existing language on self-defense and military action and the Security Council's role as the ultimate authority on the use of force. It thus does not fully acknowledge the legitimate differences that states may have about the imminence of threats in the age of terrorism. Nor does it confront the problem of how states should proceed with their "responsibility to protect" if the Security Council cannot agree.