Most of the intrusive monitoring of the government is too fulfill the "common defense", not ...
The main idea of a lot of opponents of things like surveillance is that "common defense" is actually a projection of the insecurities of some people. In the 1950s you can look at McCarthyism that swept the nation. When there is heavy emotion attached to an issue that is a sure sign that it is not being viewed accurately.
With regard to this issue of monitoring to prevent "security threats", a good historical example is the normal past treatment of what are now called terrorists. Throughout history tempered leaders have not attacked "smaller minded" people, they have let them develop. But when the smaller minded people get power they insist on using technology i.e., guns, surveillance etc to "contain the threat".
Pakistan used to keep its tribal area apart from the world, let those people develop at their own healthy pace. Then some silly numbskull got the idea to go in there and force them to play war games with monkeys from other countries. Now the weapons sellers own the narrative.
These are needed for any individuals in a libertarian society to have their freedom....
Sorry that my tone was snarky. I don't agree with your definition of libertarianism, but I know many libertarians would side with you.
What exactly is the value that your libertarian army would be defending?