I remember learning somewhere that after the US Civil War (or perhaps the War of 1812) that people started saying "the United States is" instead of "the United States are" as a result of the new unity proven in the defense against secession (or the British). I haven't seen this factoid since though. Is there any truth to the statement, is it feelgood propaganda pushed by contemporary media or modern textbooks, or is it my own creation?
The Declaration of Independence used the phrase "these united States of America" (capitalization intentional). Just as the colonists before the revolution did, people often saw themselves identified by their state first. There was a push in the 1860s towards unity of the nation, today we particularly point to the Gettysburg Address as an example of this. Referencing a singular nation, Lincoln helped to change it to the common phrase "The United States of America".