[The] rule of law doctrine of "imminent threat" is not a necessary prerequisite of justified self defense in all cases. As I discuss in The Structure of Liberty, what is needed to justify self-defense in principle is a communication of intent to invade rights in a context that suggests its seriousness. A communication constitutes a threat that violates the rights of another if it puts him in reasonable fear of being the victim of a battery or worse.
The example I give in SOL is of someone, let's say it is me, who takes a full page advertisement in The New York Times announcing my intention to murder [a certain person] at some time within the next 7 days. Assuming it is not obviously a joke, and that I apparently have the means to carry out my threat, would [that person] have to wait until I came around to his house and made an overt threatening act, which ordinarily is required by the law of self defense? Given the nature of this "standing threat," need there also be a showing of imminence?
I think under these special circumstances, [that person] should not have to wait until I chose a time and place convenient for my attack but could seek me out to preemptively defend himself against me at a time and place of his convenience. In SOL I call this "extended self-defense." What makes this hypothetical unusual and unrealistic is the unambiguously objective manifestation of intent in the advertisement. The advertisement is what constitutes the threat that is the necessary condition of self defense and no further overt act is required. Under these circumstances [that person] is entitled, in my view, to "preempt" my attack before I ever perform an act that can be deemed "imminent" (like produce a weapon and point it in his direction). But this is so abnormal a hypothetical (criminals do not normally advertise their intentions) that it does not undermine the normal importance of imminence or to the law of self defense.
But advertisements and imminent acts (like massing armies on borders) are not the only ways to communicate a threat. So would speeches coupled with less normally obvious behavior. If the content of these other communications are sufficiently clear, then self defense would be warranted even in the absence of an overt act that constitutes an imminent threat. So "imminence" may not be a requirement of even a defenseist foreign policy (assuming that a [defenseist] foreign policy is logically entailed by libertarianism, which I doubt). What is required is a threat. ...
None of this however, is to argue that a military invasion is always (or ever) a good foreign policy. Many libertarians are "noninterventionists" who seem to oppose almost any military invasion outside the territory of the US on the ground that the unintended consequences of such actions are likely to be terrible, as indeed they often are.
My original point was simply that this type of noninterventionism, whether right or wrong, does not follow from Libertarian principles as some of its adherents apparently assume. It is more a pragmatic judgment of the sorts of rightful actions that will or will not yield good consequences. This judgment could lead to certain principles of foreign policy, but these should not be confused with Libertarian first principles. In addition, while I respect those who hold to this position, it tends to ignore the unintended consequences of nonaction, which can be just as harmful. Unintended consequences is a concept that, logically, runs in both directions. ...
Finally let me hasten to add that, though I have thought a lot about Iraq as a citizen, with these posts I have only just begun to think about the relationship of Libertarianism with foreign policy. I am completely open to being persuaded that this analysis is completely wrong (as well as to encouragement that I am on the right track). Indeed, I had hoped that, by raising the issue, someone else would [do] the [heavy] lifting and save me the trouble.