Who Else Would Do It? Who Else Could?

In re: to my Bob Barr/Libertarian post, Anne of Idaho writes:

People lose sight of the fact that our military strength does more than protect the USA . We patrol the waters in the Mediterranean in order to keep the region stable so that all countries can benefit from the peaceful movement of oil and other goods. Our presence in many areas keeps traditional foes restrained. And our military strength enables other countries (think EU) to have tiny military budgets. They know we are there. What would they do if we weren’t?

They would be royally screwed, me-thinks. Not that we say these things out loud. It wouldn't be politically correct to point out the world's dependency on us, nor to state that there is no other nation on earth who does so much, for so many, with so little thanks - or reciprocity. But, as Ronald Reagan once said, "It is amazing what you can accomplish if you do not care who gets the credit."

Getting back to how all this relates to Libertarianism, though, I still ask: what is the logical-philosophical justification for the party's Isolationist ideas? I need a Libertarian to tell me.