A commenter on the Republic/Democracy thread mentioned the shift from "United States" being seen as a plural noun -- e.g., the Constitution's "Treason against the United States, shall consist only in levying War against them" or the Thirteenth Amendment's "Neither slavery nor involuntary servitude ... shall exist within the United States, or any place subject to their jurisdiction" -- to the modern treatment of the "United States" being singular ("the United States is ...").
This reminded me of an earlier post, which cites an item by Mark Liberman (Language Log). Liberman was commenting on the assertion that,
Before the war, it was said "the United States are." Grammatically, it was spoken that way and thought of as a collection of independent states. And after the war, it was always "the United States is," as we say today without being self-conscious at all. And that sums up what the war accomplished. It made us an "is."
Liberman investigated -- see the link above for the start of his investigation -- and discovered that, according to Minor Myers, Supreme Court Usage and the Making of an 'Is', 11 Green Bag 2d 457 (2008), usage changed quite gradually, at least in the Supreme Court. To quote Myers,
This survey examines use of the phrases "United States is" and "United States are" in opinions of the United States Supreme Court from 1790 to 1919. It demonstrates that the familiar claim about the timing of the change is not accurate. In the Supreme Court, the plural usage – "United States are" – did not end with the Civil War. Although patterns of usage changed abruptly in the 1860s, justices continued to use the plural form through the end of the nineteenth century. Indeed, the plural usage was the predominant usage in the 1870s, 1880s, and 1890s. Only in the beginning of the twentieth century did the singular usage achieve preeminence and the plural usage disappear almost entirely....
The Civil War does not appear to have altered the Supreme Court's usage in a fashion as dramatic as [some] have suggested. In the 1860s, the usage pattern shifts away from "are" and toward "is," and it is during that decade that usage of "is" first predominates. But the change is not wholesale – "are" and "is" were used roughly equally in the 1860s. In the following decade, Court usage reverted back to antebellum patterns. For the remainder of the nineteenth century, plural usage predominated in Supreme Court opinions, though by slowly declining margins.