State borders are aggression.

Ownership is the right to decide how a thing is used. So when a person claims to have a right to tell you that you can't use some part of nature, whether it's land, animals, plants, air or water, they're claiming ownership of that thing. Property can come into rightful ownership in two ways. By original appropriation, or by voluntary exchange from someone who did take an unowned resource from nature. If I claimed ownership of the entire world, people would call me crazy. They'd know that I had no right to own it, because I hadn't homesteaded it. I hadn't made use of it, and much of it is owned by others.
So if I can't claim to own the world, why can a relatively small group of people claim to own a territory that also has rightfully owned property inside? This simple concept may be the most ignored and important fallacy in the world. If a person, or group of people haven't come to own something through one of the two valid ways, they have no right to decide how a thing is used. That goes for countries.
This doesn't stop people from claiming to own the property of others, and unowned land. It's much the same as a mugger claiming that they should control the money in someone else's wallet. It's aggression. Only private property owners have the right to exclude others from that property. Every use of force to prevent peaceful people from emigrating is a violation of human rights. Capitalists believe that the market should decide most things, but this is one area where many of them abandon their principles, and turn to socialism in fear that their ideas can't stand the contest of freedom. I hope that one day, humans realize that we're not supposed to be a domesticated species.