[jimmyk]

>Broadly controlling human migration at the border is a relatively recent thing (as in, just over a century).

When prior to industrialization would mass migration (especially of another ethnic group) have been seen by the current inhabitants as anything other than an invasion?

>Taking US as an example where this sort of thing is a controversial political topic today - in most of the 19th century, if you had access to any means of transportation necessary to get there (including walking across the border from, say, Mexico or Canada), that was all you needed to become a legal resident. Yet the country did just fine.

The immigration restrictions of the late 19th/early 20th century were a reaction to immigration numbers having risen beyond the point that the current inhabitants were comfortable with. Yes, controls were lax before then but that was a reflection of 1) the relatively low degree of conflict over resources (a result of industrialization), 2) the similarity of ethnicity/culture between the current inhabitants and the immigrants (they were predominantly Christians of European ancestry -- Chinese immigration was not viewed so favorably), and 3) comparatively low numbers of immigrants before ~1870, and especially before 1850.