Mark Liberman (Language Log) comments 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 investigates (see the link above for the start of his investigation), and discovers that, according to Minor Myers, Supreme Court Usage and the Making of an ‘Is’, 11 Green Bag 2d 457 (2008),
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.