People just make this stuff up, any rationalization for hatred. It's not hard to see how immigration has worked, how Europe integrates with itself (a continent that fought endless wars before the proto-EU was born), and how the US is a country of immigrants. Rather than repeat it all, see:
https://news.ycombinator.com/item?id=44092557
Excerpt, from Ben Franklin:
"... Why should Pennsylvania, founded by the English, become a Colony of Aliens, who will shortly be so numerous as to Germanize us instead of our Anglifying them, and will never adopt our Language or Customs, any more than they can acquire our Complexion. ..."