History: United States Of America, 1900-1930

The standard way to refer to a citizen of the United States is as an American. Though United States is the formal appositional term, American and U.S. are more commonly used to refer to the country adjectivally ("American values," "U.S. forces"). American is rarely used in English to refer to people not connected to the United States.
The phrase "the United States" was originally treated as plural—e.g., "the United States are"—including in the Thirteenth Amendment to the United States Constitution, ratified in 1865. It became common to treat it as singular—e.g., "the United States is"—after the end of the Civil War. The singular form is now standard; the plural form is retained in the idiom "these United States".