If you spend enough time in libertarian anarchist circles, you’ll come to recognize various factions. Two libertarian anarchist factions that like to fight with each other are the advocates of open borders (more specifically the advocates of abolishing governmental borders) and the intellectually inconsistent advocates of closed borders.
Whereas advocates of open borders recognize the State as wholly illegitimate, advocates of closed borders see the State as semi-legitimate. On the one hand, it steals from them (and everybody else), which makes it a violator of private property rights and therefore illegitimate. On the other hand, it subsidizes their security (with, I might point out, stolen money but I digress) by providing law enforcers and a military. If you talk to an advocate for closed borders, you probably won’t hear them discuss the fact that the State is subsidizing them (since that would be admitting government subsidies are good and they generally claim otherwise). They’ll give several other reasons why the State is acting legitimately by controlling its borders, usually with an argument that tries to muddle private property lines with government borders, but no amount of hand waving makes the fact that they want their security subsidized go away.
Where the argument for closed borders begins to really fall apart though is when you compared government borders to private property lines. Property lines, like borders, aren’t a real thing. In the terminology of Max Stirner, property lines are a phantasm or a spook. They exist entirely in our minds, not in the natural world. However, like many human concepts, property lines can serve a purpose, which is to avoid conflict over scarce resources. Two people cannot consume the same piece of bread so to avoid fighting over a piece of bread it’s expedient to say one piece is my property and one piece is your property. Conflict is avoided so long as both of us recognize each other’s property claim.
Government borders serve a similar purpose but the resources differ. While one might think that the raw resources within a government’s borders are what it’s trying to claim ownership over, in reality governments care little about the raw resources themselves. What governments care about are the people that harvest those resources. Governments are also phantasms. They’re a concept in our minds, not a thing that exists in the natural world. The people who call themselves government, on the other hand, do exist in the natural world and they don’t like to do work. Instead, like a mafioso, they prefer to skim a little off the top of other people’s work. The individuals who call themselves government don’t want to till the fields or mine the mountains, they want to take a percentage of the wealth created by the people who till the fields and mine the mountains. To the government the only meaningful resource is the human being.
A funny thing happens under libertarianism when a human being is being claimed as a resource. Under the concept of the non-aggression principle, which is the closest thing to a common philosophical foundation most branches of libertarianism can agree on, slavery is illegitimate. One person claiming ownership over another person becomes a violator of the non-aggression principle as soon as the person making the claim attempts to assert their claim. Governments continuously assert their claims of ownership, usually under various euphemisms such as enforcing the law, over people.
Since one human being is incapable of doing two things at the same time, governments periodically come into conflict with one another over what they want a group of human beings to do. What happens when one government decides that it wants a group of humans to farm its territory while another decides that it wants them to mine its territory? Conflict. To avoid conflict the individuals calling themselves government have take the concept of private property lines and relabeled them national borders. Governmental borders quite literally exist to avoid conflict over human property. Since enforcing a claim of ownership over another human being is considered illegitimate under libertarianism, supporting the division of human property cannot be consider legitimate under libertarianism in any consistent manner.