After: the United States is..."
An apocryphal fantasy repeated by, among others, Shelby Foote who should know better.
As early as the 1820s, "is" was used about 1/3 of the time.
By 1860 the use of "is" had been rising for 20 years.
In fact the change from "are" to "is" was grammatical, reflecting a change in American English usage, not political perspectives.
Here is an example, from the 13th Amendment (1865):
- "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction."
"An 1895 column in the Indianapolis Journal defended the usage of Secretary of State Richard Olney, who preferred 'the United States are.'
The writer insisted that this was correct usage on grammatical grounds: 'Thoroughly as one may believe in the idea of nationality, one cannot ignore the structural principles of the English language.'
As late as 1909, Ambrose Bierce was clinging to this grammatical defense of 'the United States' as plural."
It's true that "is" became more common after the Civil War, but not necessarily because of it.
We can note the 50% mark ("is" v. "are") was not reached until around 1885.
Here is a study of historical uses of "is" versus "are":
