HSPD-8 directed DHS to lead a national initiative to develop a National Preparedness System-a common and unified approach to strengthen the preparedness of the United States to prevent and respond to threatened or actual domestic terrorist attacks, major disasters and other emergencies. The requirements of HSPD-8 led to the National Preparedness Guidelines, which was developed to provide the means for the Nation to answer three fundamental questions: (1) How prepared do we need to be? (2) How prepared are we? (3) How do we prioritize efforts to close the gap? HSPD-8 also required DHS to develop mechanisms for the improved delivery of Federal preparedness assistance to State, tribal, and local governments and to strengthen the Nation's preparedness capabilities. Fifteen National Planning Scenarios were developed to illustrate the range, scope, magnitude, and complexity of incidents for which the Nation should prepare. Using this wide range of possible scenarios, including terrorism, natural disasters, and health emergencies, helps reduce uncertainty in planning. (FEMA, National Incident Management System (FEMA 501/Draft), August 2007, p. 11)