I’m not sure if you mean this literally or metaphorically. While it’s a catchy way of metaphorically encapsulating changes in popular thought before and after the Civil War, it’s certainly not strictly true. The two usages have different semantics, both of which are useful in different ways.

William Cobbett, A History of the Protestant Reformation in England and Ireland, 1832, p. 197:
The United States of America is a very happy country.

William H. Seward, The Works of William Seward, 1853, p. 415:
Her argument is, that ” the United States of America is a body corporate, distinct from the states as political bodies, and capable of holding real and personal property;”

When talking about the country as a whole, the singular clearly makes sense, because the several states formed one federal nation, which obviously was true both before and after the Civil War.

George Bancroft, History of the Formation of the Constitution of the United States of America, 1889, p. 331:
The United States of America are not only a republic, they are “a society of societies,” “a federal republic.”

John Bouvier, A Law Dictionary, Adapted to the Constitution and Laws of the United States, 2004, p. 591:
The United States of America are a corporation endowed with the capacity to sue and be sued, to convey and receive property.

When referring to common properties of the States together, the plural seems most fitting. In the 2004 example, I’d actually expect the singular, so it’s particularly notable and interesting that this law dictionary chooses to use the plural.

I know, I know. I’m a spoilsport.

They compose entirely new scores for the silent films they perform (both shorts and features), and perform them on a combination of traditional and off-the-wall instruments (think bedpans and giant springs, etc). I loved it. They sell DVDs, but I never bought any, and I think the live show is probably really where it’s at. One I went to was a series of shorts, including the pre-sound Laurel and Hardy short Big Business (Google Video, YouTube part 1 and part 2), and One Week (Google Video, YouTube part 1 and part 2), a Buster Keaton classic. I also saw their performance of the Harold Lloyd feature Speedy:

I’m thrilled to discover that some of their work is on YouTube. Just search “Alloy Orchestra” and you’ll find a few Buster Keaton films with their accompaniment. Cool!

One of my friends in school, who had lived in Arkansas for a dozen years or so before moving to down, had a license-plate-size sign he kept on the dash of his truck, depicting an angry-looking cartoon confederate soldier holding a confederate flag, with the words, “Forget? Hell!”

His older brother had a big belt buckle with the confederate flag in the shape of the U.S., and the words “The South will rise again”.

And don’t forget that the South Carolina capitol building still flies a confederate flag to this day.