Oh's Law (On Assimilation)
If a host nation is to allow foreigners into their land, assimilation over time ought to be the foremost political goal regarding them. But we recognize that this takes time, often spanning multiple generations. Thomas Aquinas remarked how when foreigners wished to be admitted entirely to the fellowship and worship of the Jews, "they were not at once admitted to citizenship: just as it was law with some nations that no one was deemed a citizen except after two or three generations, as the Philosopher says. The reason for this was that if foreigners were allowed to meddle with the affairs of a nation as soon as they settled down in its midst, many dangers might occur, since the foreigners not yet having the common good firmly at heart might attempt something hurtful to the people." Foreigners always bring with them (to no fault, for this is natural) their own customs, values, culture, and religion; thus it is prudent to restrict their political and social influence in order to protect the way of life of the host nation.
I have often stated an informal heuristic for gauging whether or not a foreigner has been sufficiently assimilated into his host nation. We can call it "Oh's Law" and state it something like follows:
Oh's Law: A foreigner (or the descendant of one) has been sufficiently assimilated into his host nation if he would have no hesitation fighting for them if they were to war with his (ancestor's) motherland.
To me, this is perhaps the most telltale indication that a man has sufficiently severed all of the attachments of his (ancestor's) motherland that could reasonably call into question his ultimate loyalties. The word "sufficiently" is key, for many attachments are to be tolerated and even encouraged (to honor one's ancestors is a duty contained in the fifth commandment), but none that would sabotage the good of the host nation. As Aquinas noted above, I also do not believe this will happen until the second or third generation, if not even later. I would say that I myself (second generation) do not pass Oh's Law.
There are certainly many other things that would need to be true to consider a man sufficiently assimilated. But in my opinion, this is a simple, clear, and important qualification to meet.