[I am trying to explain what I think is a conceptual confusion by the "Open Borders" people. Unfortunately, my brain isn't very clear either.]
Land can't move, of course. But borders can. We can't move land across the borders, but we can move borders across the land.
So if half the people in country B want to move to country A, there are three ways we could satisfy their desire:
1. Move the people across the border.
2. Move the land and people across the border, by letting country B annex half of country A.
3. Move the land and people across the border, by letting country A annex half of country B.
Which of those three would be better? Are all three equally feasible? Why would people object more to some than to others? Why would one be more morally acceptable than the others?
Would the people earn more if they moved to country A?
Would the land earn more if it moved to country A?
Here's a very abstract way of thinking about it:
There are three things: people; land; and law. Please understand "law" in the very widest sense, to include not just written law but unwritten law, customs, and ways of life. "Institutions" might be a better word.
A country is a set of people, land, and law. What is the optimal allocation of countries? (Think of it like the optimal currency area question, except we have three things that can move in relation to each other.)
People have preferences defined over land and law. When people want to move across a border, it might be because they like the land better, or it might be because they like the laws better. Or it might be a bit of both. Their income and standard of living will depend both on the new land and on the new laws.
"Colonialism" is where people and laws move together, to new land.
"Imperialism" is where laws move to new land, and the people don't move.
"Migration" is where people move to new land, and the laws don't move.
Which one is best, and who gets to decide?
Law is produced mostly by people, and less by land.
I like consuming cars, but I don't like producing cars. Maybe it's the same for law. We solve this problem for cars, by having someone else produce the car and me paying them for the car. Can we solve the problem the same way for law too? Probably not as easily, or else imperialism would work fine.
A country is a native reserve with an army. [US English translation: a country is a native reservation with an army.] It's where people follow their traditional laws on their traditional land. The army is there to keep it that way, except when they choose to let in migrants who will also follow their laws.
I can understand that some people might want to live on a reserve.