I wasn’t sure which thread to put this in, but I think it goes better here; however, I’ll start by partly quoting a couple of posts from that other thread:
I think y’all have gone down a bit of a blind alley here. IIRC, the claim wasn’t that there’s a distinction between “the United States” and “these United States” (I’m pretty sure I occasionally still hear politicians say “these United States”, purely as a rhetorical flourish). The claim is that Americans shifted from “the United States are” before the Civil War, to “the United States is” after the war.
There does seem to be some validity to that claim. In the Constitution, for example, the plural is used (“Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies…”), which grammatical form is still found in the U.S. Code, but definitely sounds archaic these days. To semi-randomly pick one of the documents Tom linked to in that other thread, the Treaty of Guadalupe Hidalgo, a careful read still shows that was the case in 1848: “The United States do furthermore discharge the Mexican Republic…”; NOT “The United States does…”. (Contrast this with the Treaty of Peace Between the United States and Spain in 1898: “The United States will adjudicate and settle the claims of its citizens”; NOT “the claims of their citizens”. The pre-Civil War Treaty of Guadalupe Hidalgo by contrast at one point refers to “The United States, exonerating Mexico from all demands on account of the claims of their citizens…”)
It’s an interesting linguistic point. I do think some people read way too much into it. I recall a claim right here on the SDMB within the last month that before the Civil War the U.S. was more like “the 1970s European Common Market” rather than a true state, which is not at all tenable. (The 1970s EEC wasn’t going around waging war on and then annexing large chunks of nearby countries; even linguistically, the Treaty of Guadalupe Hidalgo, although using plural forms to refer to the United States, also consistently refers to “them” as a singular republic, not as a loose alliance or confederation of republics.)
More to the point, whether the term “United States” is regarded as grammatically plural or singular, there was manifestly a strong sense of nationhood and of a national identity as “Americans” before the Civil War; if there hadn’t been, there wouldn’t have been a (long, bloody) Civil War in the first place. Nobody is likely to start a shooting war over Brexit.