Are Frontiers Always Free?
Many Americans (and most American science-fiction fans) think the fact that the U.S. is a free society is because it is a frontier society. You can see a typical example of this reasoning in “Margin of Profit” by Poul Anderson. On the contrary, there have been unfree frontier societies. We can start with the southeastern U.S. in the early 19th century and continue with such examples as the State of Qin or the fact that Russia institutionalized serfdom just when it was becoming a frontier state. Apparently the labor shortage characteristic of frontier societies encourages the extraction of labor by force (ObSF: the Draka novels by S. M. Stirling).
I suspect that if any external feature is associated with free societies, it is ease of defense. If we look at the parts of Western civilization that have been free as a matter of ancient tradition, they include islands, swamps, and mountains. There are two reasons for this: 1) If you live in a rugged areas, someone else's king can't enslave you. 2) Your king can't enslave someone else. In non-rugged areas, eventually the most powerful governments will be those best equipped to conquer their neighbors.
I've heard that The Art of Not Being Governed by James C. Scott has a similar thesis but I haven't read it.
A final note: If American freedom were only a matter of having a frontier, you could plausibly argue that it is obsolete. In the real world, that is not necessarily the case.