Long article. Below is the basic idea.
https://www.lewrockwell.com/2018/07/hans-hermann-hoppe/immigration-and-libertarianism/
…But on what grounds should there be a right to un-restricted, “free” immigration? No one has a right to move to a place already occupied by someone else, unless he has been invited by the present occupant. And if all places are already occupied, all migration is migration by invitation only. A right to “free” immigration exists only for virgin country, for the open frontier… Read the rest of this entry »