United Nations Security Council resolution 678 (1990) authorizes the use of all necessary means to enforce United Nations Security Council resolution 660 (1990) and subsequent relevant resolutions and to compel Iraq to cease certain activities that threaten international peace and security, including the development of weapons of mass destruction and refusal or obstruction of United Nations weapons inspections in violation of United Nations Security Council resolution 687 (1991), repression of its civilian population in violation of United Nations Security Council resolution 688 (1991), and threatening its neighbors or United Nations operations in Iraq in violation of United Nations Security Council resolution 949 (1994).
Fun with legalese:
United Nations Security Council resolution 678 (1990) authorizes the use of all necessary means to enforce United Nations Security Council resolution 660 (1990) and subsequent relevant resolutions and to compel Iraq to cease certain activities that threaten international peace and security, including the development of weapons of mass destruction and refusal or obstruction of United Nations weapons inspections in violation of United Nations Security Council resolution 687 (1991), repression of its civilian population in violation of United Nations Security Council resolution 688 (1991), and threatening its neighbors or United Nations operations in Iraq in violation of United Nations Security Council resolution 949 (1994).
http://en.wikipedia.org/wiki/Legality_of_the_Iraq_War