Richard Epstein masterfully explains why libertarianism does not imply isolationism.
The just state should, in my opinion, protect private property, promote voluntary exchange, preserve domestic order, and protect our nation against foreign aggression. Unfortunately, too many modern libertarian thinkers fail to grasp the enormity of that last obligation. In the face of international turmoil, they become cautious and turn inward, confusing limited government with small government. Unwisely, they demand that the United States keep out of foreign entanglements unless and until they pose direct threats to its vital interests—at which point it could be too late.