> Why do you think it was OK back then? Have you talked to any Native Americans about the effects of uncontrolled migration on their tribes? Plainly put, it was an unmitigated disaster and a genocide. They used to control the whole continent (but divided between various tribes of course, that sometimes didn't get along); now their numbers are puny and they're corralled in some shitty reservations.

In 1776, the population of the United States was mostly British. Most of the population of the United States today is descended from non-British European immigrants. But they didn't wipe out the existing British population – they just assimilated into it. The genocide of indigenous people was not a consequence of "immigration," it was a consequence of conquest. I'm against immigrants waging war against and driving out existing populations of the countries they come to, but that is not what is being discussed.

> Within a country, it's easy: people have criminal records, and can't just skip town and go to the next town and assume a new identity. (Centuries ago, however, they did.) Between countries, it's not so easy: countries don't share their criminal data that easily.

Ok, so then countries should work to start sharing criminal records and then they can have open borders, right?

> That's why we have passports: it's an identification document that shows the destination country that the person is not a criminal and is allowed to travel

A passport effectively only allows you to travel if you come from a rich country. If you come from a poor country, you need to apply for a visa. But in either case, you still need to go through a controlled border, and you're only allowed in the country for a limited time. If the concern is really about letting in criminals, and a passport or visa alleviates that, why is it so limited? And why aren't countries working to share criminal records so they don't have to do this song and dance? Hint: it's not really about crime.

> Why is that cruel? If 100M people suddenly decided to move into Andorra, how exactly do you think the country is going to pay for social services for them?

Andorra is part of the Schengen Area – there's no reason why they would all stick around in Andorra. If 100 million people came into the Schengen Area, they would get jobs and pay taxes to fund social services just like everyone else. But you ignored my main point – if you're convinced that immigrants will consume more social services than they pay taxes, why not just let them in but make them ineligible for social services? It's not really about social services either.

The reason that first-worlders don't want third-worlders coming to their countries is because (a) they don't like their skin color, (b) they don't like their culture, and (c) they'd rather have people live in extreme poverty far away than in a reduced level of poverty where they can see it, the same reason they're against building affordable housing in their own neighborhoods. When Western countries started implementing immigration controls, they were very open about this, but since the 1960s it's become fashionable for them to pretend otherwise.

Now, you might object: if that's the reason, why don't all the rich Western countries have open borders with each other? I mean, most of them do (Schengen), but still, why doesn't the USA have open borders with Canada? And the answer is: there's no reason at all. I can't think of any argument against open borders with Canada. The only reason it's not the case is because if all the white countries had open borders with each other, they wouldn't be able to lie about the reason anymore.