In comparison with the nearly two-and-a-half-centuries-old United States, many countries in the world today are new, some of them created in the relatively recent past. Yet many of these states, new as they may be, are undergirded by peoples who had a pre-existing sense of their common ancestry, their tribal and blood connections, by which they meant their nationhood.
In the case of the United States the process was reversed. In some sense we have never become a nation, and today, with people from all over the world gathered within our borders, we can never be a nation in any traditional meaning of the term.
In the present this peculiarity of American nationhood, this lack of a common ethnicity, may be our saving grace. It may turn out to be an advantage in the 21st century, dominated as it is by mass immigration from the south to the north and east to west. It certainly enables the United States to be more capable than other countries of accepting and absorbing immigrants.