In Democracy in America, Alexis de Tocqueville said,
"The United States have no metropolis; but they already contain several very large cities."
It was common in de Tocqueville's day to use plural verbs to speak of the United States -- "The United States are a great source of inspiration to those who love Liberty Under God" -- because it was a union of sovereign States, and the Constitution acknowledged the sovereignty of those States.
The Declaration of Independence says that the United States are
Free and Independent States; and that as Free and Independent States, they have full Power to levy War, conclude Peace, contract Alliances, establish Commerce, and to do all other Acts and Things which Independent States may of right do.
This means the states have "assumed a separate and equal station among the Powers of the earth," and Britain, Mexico and China have no authority to write or re-write the criminal codes in these united States. Nor does the Federal Government in Washington, D.C.
In our day the federal government has ignored the Constitution, the original intent of its Framers, and the enumerated powers it has under the Constitution, and has almost completely eliminated "states' rights." It seems that its main purpose is to protect its own power, and further centralize power over the states. "The United State is no longer an inspiration to those who love Liberty Under God."
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."
17. Zimmer, Benjamin (2005-11-24). "Life in These, Uh, This United States". University of Pennsylvania—Language Log. http://itre.cis.upenn.edu/~myl/languagelog/archives/002663.html.
(retrieved Aug. 9, 2009)