In a free society It's supposed to keep "them" out, and we have not been doing a good job of late, In a Communist one, it is supposed to keep "them" in, they do a better job of it, of course, as they are not afraid of going to any length to achieve their border security,
Hitler just needed "living space" for his people... He was so bold as to take it. That worked out well.
When my grandparents came here from Sicily, they immigrated the legal way - through Ellis Island and didn't bring waves of crime and drug smuggling with them the way illegal aliens from south of the border are. My grandparents also didn't sign up for every type of government assistance - they had to get jobs and somebody in this country was responsible for them. Or they got deported.