The best offense is a good defense?
Robin is, to be sure, correct that our military does not perform the traditional libertarian task of protecting our borders from invasion. Instead, it protects our interests abroad--and does so well enough that Europe, for all its complaining, is happy to free ride. This suggests that we are, for all our faults, doing a fairly good job of protecting the broader interests of liberal capitalistic democracy. A world in which China and Russia and Iran were using their military might to protect their interests, while we squatted behind our borders, would be possibly a nobler one, but I find it unlikely that it would be a better one in any utilitarian sense.