For what it's worth, I think Slobodan has the stronger historical argument—and one that is specifically relevant to this thread.
The nation-state is primarily a continental European concept, which derives from the sorting out of political boundaries after the religious wars of the 17th Century. That's when the idea of conflating ethnicity (nation) and jurisdiction (state) became widely accepted as a defining principle in Europe. Not necessarily elsewhere, although I think there are close analogues in Africa and parts of Asia (e.g., Japan and Thailand).
Americans, north, south, and central, have always been different. The concept of "nation-state" never made sense here. Although, admittedly, the term has recently been adopted by some right-wing populist commentators.
Even in the pre-Columbian era, while there were what we would today consider to be ethnic differences—tribal and linguistic—between the non-nomadic indigenous populations, as far as I am aware most of them never had anything resembling a modern theory of territorial boundaries. And, unquestionably, after the Europeans arrived almost all the American colonies and countries that evolved in this hemisphere were multinational (i.e., multi-ethnic).
There was a brief period after the united states declared their independence from Great Britain (capitalization and emphasis intended) when arguably you could claim there was a correspondence between principal ethnicity (English and Scottish protestant) and political boundaries. But only in some of the states. Not in Maryland (founded as a Catholic colony), or New York, New Jersey, and Pennsylvania (with their large Dutch populations and, in Pennsylvania, a considerable Quaker minority), or in Vermont and New Hampshire (quite French in the northern parts of both states). Not to mention the Amerindian populations, or the Africans, enslaved and free, or the Germans, Irish and other "foreigners" who started arriving in large numbers even before the adoption of the constitution of 1789.
Same thing in Latin America, where in most countries the mestizo population tends to be much larger than the "pure" descendants of the colonizing Castilians or the other European immigrants who arrived more recently.
As for borders, in North America they have traditionally been very porous. My wife and I used to drive into Canada without being asked for any identification by the Canadian inspectors, and to return with no more documentation than our U.S. drivers' licenses.
I don't recall for certain, but I think we always took our passports with us when we visited border towns in Mexico. But I have a very distinct memory of returning to the United States late one afternoon in Nogales, and being impressed by the rapidly-moving stream of Mexican day workers walking back in the other direction—almost all of them carrying bags filled with goods they had purchased at Walmart, Target, or CVS. I rather doubt that all of their documentos were carefully checked when they returned for work the next morning.
