There is a long discussion of this in Ken Burns's classic documentary series, The Civil War. At least according to that work, the singular verb came to be used after the US Civil War, are the country cam to be thought of more as a single unit. I wonder how the Estados Unidos de Mexico is discussed in Spanish.
in the Spanish course I had to use the plural verb conjugation when talking about the United States. I can't remember the sentence exactly, something like "los estados unidos son muy bonitos" maybe. But yeah, that tripped me up here because I wrote "The United States are" instead of "the United States is" haha