I don't believe that the libertarian philosophy dictates a foreign policy. In particular I don't think you can derive isolationism from libertarianism. I'm anti-interventionist, but I'm not an isolationist. I don't believe we ought to go without armaments. I'm sure we spend more money on armaments than we need to; that's a different question.
I don't believe that you can derive from libertarian views the notion that a nation has to bare itself to the outside without defense, or that a strong volunteer force would arise and defend the nation.
~ Milton Friedman, "Best of Both Worlds," Reason, June 1995