How far away must a robbery occur so that a Libertarian doesn't care about it?
But before I get to that, I want to address what Kleiman wrote. Through a short (but correct) line of reasoning he concludes that war is impermissible in strict Libertarianism due to the collateral damage that is wrought. He comes to this conclusion:
There are two possible conclusions here: either (1) war is always wrong, or (2) Libertarianism as a moral philosophy (as opposed to the libertarian tendency in politics) is not merely false but transparently silly, since no actual group of people could live under Libertarian principles unless some other group of people did the dirty work of collective self-defense for them.
I think Kleiman would be hard pressed to find any one person, let alone an entire school of philosophy that would endorse the death of innocents in isolation. By Kleiman's style of ignoring-the-context-of-battle reasoning, all philosophies (save maybe that of Hannibal Lector) are insanely pacifistic.
In addition, Kleiman misinterprets Barnett's version of Libertarianism, confusing Barnett's version with the anarchic fringe of the Libertarian movement. In the abstract of his recent paper, Barnett says the Libertarian minimalist approach is "enforcing only the natural rights that define justice," and that Libertarianism is a "A compromise, as it were, that makes civil society possible." Barnett says in his paper (pg. 15) that certain "perfect" rights "create an enforceable duty in others, as in 'I have a perfect right to this land.'" (an imperfect right is something like "I have the right to like chocolate ice cream better than strawberry."), so some coercion may be justly employed to force people to perform their Libertarian civic duties in Barnett's interpretation.
With regards to Barnett's question, I have concluded that there would be no significant difference in Libertarian approaches to foreign policy from that of either conservatives of liberals. I assert that they all follow basically the same foreign policy, with differing degrees of skill depending on the individual (the war in Iraq was not conservative, it was stupid). The foreign policy of any systematic political philosophy is this:
1: While out of power, use the populist appeal of isolationist non-intervention to undermine your domestic enemies.
2:When in power, use your power (while skillfully balancing between realpolitik and idealism for maximum effectiveness) to aid intellectual fellow travelers across the globe.
Given that American political ideologies are very similar compared to the extreme ideologies that flourish in the areas of the world that are most volatile, there is rarely large dispute over which side to support in a given foreign policy issue, only dispute as to whether intervention is merited (see #1). It is very rare indeed that the ideological separation of American liberals and conservatives have impacted on our foreign policy; some examples of this are the Mexico City policy and possibly support to Aristide's government in Haiti (though I know that's debatable and I am not educated well enough on that issue to form an opinion).
The reason a true Libertarian must be internationalist in his outlook is the same as the reason he must concede duty to enforce Libertarian ideals in his own country. I'll describe a hypothetical case. Suppose we have three countries arraigned in a one dimensional geography where A is to the west of B which is to the west of C. A and B are good Libertarian countries, while C is an expansionist Communist dictatorship. If B's shared border with C forces them into many costly wars to contain the expansionism of C's dictator, A would be getting a free ride off of B's military, effectively transferring B's wealth to A. Clearly Libertarian society would not work if every individual could opt out of their duty to enforce the "perfect" rights of others while they are so fortunate that their own rights were not presently being violated. Similarly, in a Libertarian international order, each (Libertarian) country must protect the rights of others in that an aggressive country of non-Libertarians is a threat to all Libertarian societies.
This line of reasoning can be extended to Libertarian rebel groups inside of a non-Libertarian regime in that all individuals benefit from their activities that undermine the aggressive power of the coercive regime. A violator of natural rights is a violator of natural rights even if he is far away. A rights violator is a potential danger to everyone, not simply his current victims.
The nature of geo-politics used to be that there were a very small number of actors that can effect any given individual actor directly at all (his neighbors and possibly one or two naval superpowers). This is in fact the only conceivable reason for there to be a difference in the rules governing conduct of nations and conduct of individuals. "Defensism" seems to be an application of this idea. By "Defensism", exactly how long would Britain have stayed out of WWII? Though my knowledge of this subject is less thorough, would Wellington have been at Waterloo under "Defensism"? The events of September 11th demonstrate that volatile aggressors can have world wide reach.
I should just mention that Barnett seems to advocate granting fifth-amendment-suspending arrest powers to every foreign government on the planet in his discussion of NGOs (terrorists).