The Bush Doctrine of Preemptive Self-Defense

What is the difference between mere aggression and preemptive self defense?  Can you really permissibly “defend” yourself against an attack that hasn’t even begun?  How does preemptive self defense differ from preventive war, from humanitarian intervention? John and Ken do not pre-empt their questions to George R. Lucas Jr., Professor of Ethics and Public Policy at the U.S. Naval Academy.