From the United States "are" to the United States "is" -- Note for a lecture, "E Pluribus Unum? What Keeps the United States United

"Before 1861, the two words 'United States' were generally rendered as a plural noun: The United States are a republic.' The [Civil] war marked a transition of the United States to a singular noun. The 'Union'  also became the nation ... "

--Battle Cry of Freedom: The Civil War Era (1988) by James M. McPherson