Another problem is a legal one: citizenship. In the United States, anyone born on American soil, even a child of illegal immigrants, is considered a citizen, because that is how our courts have chosen to interpret the Fourteenth Amendment. But not that is not the case in every country. In Germany, the children and even the grandchildren of Turkish “guest-workers” are not considered German citizens, because in German law, citizenship is based on jus sanguinis, right of blood – you can claim German citizenship if your ancestors were ethnic Germans, or otherwise recognized as citizens. I really don’t know which arrangement is more common. But unrestricted immigration into any country that determines citizenship by jus sanguinis or some similar rule might produce a very large population of resident noncitizens, who might have most or all of the civil rights of citizens, but would be denied the right to vote, hold public office, or participate in the political process. Before long, the noncitizen population might even form a numerical majority. And that is sure to lead to civil unrest, possibly even civil war.
Underlying this problem is the fact that most countries do not have a “melting-pot” conception of national identity, as we have in the U.S. Nationalism, as it emerged in the 19th century, assumes that the “nation” is an interrelated group with common ancestors, culture and traditions. Persons of foreign origin can only be part of the national community conditionally, and on sufferance. French society, for instance, is still a long way from recognizing the children of Muslim immigrants as being truly “French,” even though non-French-speaking ethnocultural groups such as the Bretons and the Basques have been part of the French national community for centuries.
One reason Hitler was able to succeed in rounding up and exterminating the Jews of Europe is that, in most countries, Jews were thought of as a national minority, rather than a religious minority, which is the way we have generally viewed them in the U.S. Millions of Jews had lived in Poland since the Middle Ages, but the people of Poland still did not regard the Jews as “Poles,” and the Jews mostly did not think of themselves that way either.