This concept can be called "civic nationalism," and it sort of harks back to the Roman concept of "Roman citizen," wherein the Roman government granted citizenship for whatever reason to people who weren't ethnically Roman at all. That's one reason the place collapsed, of course.
Another concept is quite contrary to all this, and can be called "ethnic nationalism." That basically requires that if you want to be an American you need to have American ancestors, or at least be cousins to Americans, like Brits or Swedes or Frenchmen.
The first concept is erroneous and unstable, and the second is accurate and stable.
A guest post on the subject from KC:
There needs to be a balance between civic and ethnic nationalism, because pure civic nationalism is based on egalitarianism, which is a false god. Ethnic nationalism is hyper-collectivist and civic nationalism is hyper-individualistic.
While individualism is always preferable to collectivism, one must be careful not to overflow - hyper-individualism is, pretty much, the ultimate form of collectivism, where one views the human kind as one group. This is, as you can imagine, more destructive than any ethnic nationalistic idea.
Reality must never be ignored for the sake of feels, as bad as that may feel.
Human beings are not all the same, they differ greatly. There are differences that are very noticeable on individualistic scale and there are differences that can only be noticed on a scale where we are looking at groups of people (demographics) and not just individuals. A good analogy is high frequency noise vs. low frequency noise. Or a simpler one: if you stand right in front of a tree, you can't see the forest.
One can ignore reality, but one can't ignore the consequences of ignoring reality. Facts like the one that over 50% of all violent crime in the US is committed by members of a certain group that constitutes about 12% of the population (and they are not Amish, I can tell you that much). Demographics matter.
Another thing that civic nationalists tend to ignore is genetics. Human behaviour is, to a significant degree, determined both by nature and nurture. Every government action is reflected in the gene pool of the next generation, because it changes selective pressures on different groups. For example, the welfare state creates domesticated humans which are unable to survive without the state. In essence, every government programme is a eugenics programme (or dysgenics, depending on your perspective).
We also must consider the phenomenon of regression to the mean - unless the evolutionary pressure is applied throughout many generations (say, 10 or more), the next generation reverts to previous characteristics. Even if you have extreme vetting for immigrants from different ethnic groups and they all pass the test of cultural compatibility perfectly, their children will regress to the mean and you will end up with unrest. The fact is, as much as you want it, you will never have a racially diverse nation without an increase in crime, poverty and unrest.
This is why a nation built on purely civic nationalistic ideas will, in a generation or two, end up with the cancer of communism. Ethnic nationalism prevents that, but does so very unfairly on the individualistic scale, so the balance of the two is necessary and very difficult to accomplish.
Quibcag: Illustrated by Sensei of Denkigai no Hon'ya-san (デンキ街の本屋さん?, lit. "The Electric Town's Bookstore"), who has lots of feels.