Prior to he Civil War, the verb "are" or "were" followed "the United States" in gramatically correct construction. That is because the US was merely a loose confederation of states at the time.
Robert E. Lee often wrote or spoke of "my country"--by which, he meant the state of Virginia. Virginia was his country, according to the norms of the time. And since "My country, right or wrong" was the prevailing sentiment at the time, it is no surprise that Lee turned down a commission with the US Army in order to fight for his "country" of Virginia--even though he expressed the belief, from the very beginning, that Virginia had embarked upon a suicidal course by seceding.
I have read a number of late 18th century and early 19th century writings that refer to "these United States." As you astutely pointed out, "United States" was plural until the War Between the States settled the question of supremacy of State or Federal law.
I really, really have to wonder about that. Lee had gone to West Point. He'd run West Point. He'd fought in the US Army in Mexico. He spent much of his adult life in our country's army.
Do you really think the young Lee thought of Virginia as his country, when he took his oath to become an officer in the US Army? When he was building the West with the Army Corps of Engineers or leading men into battle in Mexico do you really think he didn't consider himself an American?
I certainly don't think Lee should have fought against his neighbors, but it's way too convenient to be able suddenly to say that one's state is one's country and one's country is nothing.