Is Pre-Emptive Self-Defense Justifiable?

Democracts are demanding that Republicans remove a Bush television ad which claims they "are now attacking the president for attacking the terrorists." Christine Iverson of the RNC states: "We have no doubt that Sen. Daschle and others in his party who oppose the president's policy of pre-emptive self-defense believe that their national security approach is in the best interests of the country. But we also have no doubt that they are wrong about that, and we will continue to highlight this critical policy difference as well as others." Is a policy of pre-emptive self-defense justifiable?

Let's further simplify the question: if X has knowledge that Y seeks to harm its person or property, is X justified in initiating force against Y? It must be said that it is justified for X to repel the invasion of Y once such an invasion of person or property has been initiated. This is the only justifiable definition of "pre-emptive self-defense" in my view. Smacking Y in the face because X thinks Y is going to steal his Twinkie does not rise to the level of justification of the use of force.

Further, and especially in the case of 9-11 and Osama bin Laden, justified use of force by the United States is limited to acts of aggression committed against the specific individuals responsible. X is not justified in killing Z in pursuit of Y. In fact, this makes X a criminal himself. Roderick Long has a post here that touches on this subject.