In my recent post "The United States as a subject", I discussed the often-repeated story that the American Civil War turned "the United States are" into "the United States is", and observed that "no one seems ever to have checked, at least not very thoroughly". It's a good thing that I said "seems", since Minor Myers has gently pointed me to his article "Supreme Court Usage and the Making of an 'Is'", 11 Green Bag 2d 457, August 2008, in which he checks this very point, very carefully, in opinions of the United States Supreme Court from 1790 to 1919.
And the answer? In the case of U.S. Supreme Court opinions, we apparently became an 'is' somewhat gradually, between 1840 and 1910. And the effect of the Civil War (or at least its immediate aftermath) was apparently to retard the change, not to accelerate it.
After citing the Shelby Foote "It made us an 'is'" quote that I also gave, Myers adds some evidence of the ubiquity of this view. Thus he quotes James McPherson, Battle cry of freedom: The Civil War era, 1988:
Before 1861 the two words ‘United States’ were rendered as a plural noun: ‘the United States are a republic.’ The war marked a transition of the United States to a singular noun.
And also William Michael Treanor, "Taking Text Too Serously: Modern Textualism, Original meaning, and the Case of Amar's Bill of Rights", Michigan Law Review, vol. 106, 487-544 (Dec. 2007):
‘United States’ was often matched with a plural verb in 1787 and consistently matched with a singular verb after the Civil War.
In order to evaluate these claims in the case of Supreme Court opinions, Myers used the following method:
For each decade in the survey period, I ran word searches for “United States is” and “United States are” through the Westlaw Supreme Court database. To eliminate false positives, I reviewed the search results to identify opinions where (1) “United States” was a subject and (2) the associated verb was “is” (or “are,” depending on the search). To isolate only usage choices made by the author, anything appearing only in a quotation from a statute, a court rule, or another case was ignored, as was anything in West headnotes. Each opinion in a particular case was treated as a separate work, and thus a case could have more than one entry if more than one justice wrote or if a justice used both “is” and “are” in the same opinion. I collected data on usage in the opinions of justices, the arguments of counsel before the court, and supplementary material prepared by the reporter of decisions (e.g., a syllabus). Except where noted, the focus of the presentation here is on usage in opinions of the justices; data on usage in other portions of the case reports appear in the Appendix.
Here are his basic results in graphical form (click for a larger version):
The Civil War does not appear to have altered the Supreme Court’s usage in a fashion as dramatic as Foote and McPherson 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.
Usage was quite clearly unsettled in the latter part of the nineteenth century. One of the most striking demonstrations of this is Justice Samuel F. Miller’s majority opinion in United States v. Lee. Justice Miller managed to compose a sentence with both usages:
“[T]he doctrine [of sovereign immunity], if not absolutely limited to cases in which the United States are made defendants by name, is not permitted to interfere with the judicial enforcement of the established rights of plaintiffs when the United States is not a defendant or a necessary party to the suit.”
He observes that some of the obvious theories about sources of variation don't pan out, at least in this data set:
Geography does not help explain this pattern. Looking at the geographic latitudes of the justices’ residences prior to appointment, there is no meaningful difference between the mean latitude for the exclusive “are” users and the mean for those who dabbled in “is.”
Politics doesn't seem to help either, at least in the obvious way:
To see whether the Civil War might have influenced usage in a different way, I isolated the usage by justices who were appointed by President Abraham Lincoln. In fact, during the period when at least one justice appointed by Lincoln was on the Court, the five Lincoln-appointed justices used “are” slightly more frequently than did the other justices.
Here's his appendix, giving the counts in different sorts of SCOTUS material:
It would be interesting to look at some other features as well — number agreement with verbs other than is/are; (which would help to increase the rather small counts from this source); the distribution of pronouns co-referential with the United States (19th-century newspapers give us examples of they, it, she, and we); what fraction of "United States" instances are subjects as opposed to modifiers or PP complements or whatever; how usage is affected by the topic, e.g. relations of the federal government to foreign governments, to the states separately, to individual citizens or companies, etc.
West's materials are definitely not accessible for automatic processing of such questions; but most if not all of the same documents are available on the web, I think, so this might be a good testing ground for the idea of automatic or semi-automatic analysis of this type.