Henley and Tacitus are, in essence, arguing about those proxy enemies. Are they really dangerous to Americans, or not? If so, how much? Etc. etc.
But this misses the point that if America followed a libertarian foreign policy - isolationism - we would not have foreign enemies; either directly on our own account or via proxy. Therefore it would not matter to us whether they are working together or not; and there would be no necessity for us to try to analyze intelligence to try to determine for every group in the world whether or it is "trying to kill us". Consider Switzerland. Nobody is writing long screeds arguing about whether or not Hezbollah is targeting Swiss citizens or not in their war with Israel. It's a non-topic. Isolationism works, at least against foreign terrorists (domestic is different; there, liberty works).
Now, Tacitus seems to be aware of the attractiveness of political isolation; and he tried to head it off via an analogy with personal crime: the Kitty Genovese analogy:
There are ample grounds for warring against these three groups within the context of strictly American interests. All three have posed, and do pose, a clear and present danger to the United States and Americans. But let us not pretend that there would be no such case for war against them in the absence of such direct threats. I would argue that it is enough that they are practitioners of jihad. Even if you don't accept that, it is nonetheless still enough that they are, by any objective standard, barbarous and evil, and perpetrating their monstrous crimes upon innocents. In my book, that merits my active opposition. I know that libertarians like Henley disagree: in their book, foreign policy must be run strictly on the Kitty Genovese principle. Which has the advantage of being simple and easy to apply, for sure. When you see the dead neighbor, though, it tends to tax the conscience.In Tacitus' mind, there is no difference between state action and individual action. If there an evil anywhere in the world, it merits Tacitus' "active opposition"; but by this he means not that he, personally, will go to try to solve the problem - rather, he means all Americans should be taxed, to pay for yet others who will go, to go do what he, Tacitus, says they should do, to solve the problem. This is not generousity as a libertarian understands it.
America should be myopically concerned with evil only in America: defending ourselves against invasion. That does not, however, mean that Americans can do nothing about non American evil. They can send (their own) money; they can volunteer themselves (not others), and go try to help. But America as a state should do nothing - solving problems in Zimbabwe is not the business of America.
There are several huge advantages to isolationism, which I think Tacitus has a dim understanding of. One is, that it's very easy to crisply delineate boundaries for it; consider by contrast the neocons' "invade the world" idea: should "we" invade North Korea? Haiti? Where is there not evil in the world, other than possibly Antartica? Yet, "we" seem to be unwilling (or dare I say unable) to invade every country in the world.
A second big advantage to isolationism is that it involves no necessary invasion of our other liberties. A peaceful nation is compatible with liberty; a nation at war is not.
And a third advantage: it's moral. Unlike Tacitus' taxation funded strategy of invade the world.
And finally, there's a big practical advantage to fighting against evil with your own time, money and life, rather than others' forceably taken: costs are internalized, which (economics tells us) will result in solutions that work better; better bang for the buck.
Of course, there's a huge downside to a moral foreign policy, from the POV of secular millenarialists like Tacitus: when you can't use force to extract resources, the scale of what you can do to push around foreign people and "fight evil" is vastly reduced; and that's especially so for "evil" that's mostly in the eye of the beholder. To those who see America as the instrument of history or God or Progress, to uplift humanity and bring about Good everywhere, this is a big problem.
It's not to me, nor should it be to any decent libertarian. You'd think that the idea of using the state to achieve secular salvation would have died with the Soviet Union - but no, only the idea of domestic socialism was discredited. War socialism is alive and well in the American Right.