This is the last episode of the Burns miniseries. It’s where Shelby Foote makes the silly claim that the result of the war caused us to stop referring to the United States in the plural and instead refer to it in the singular. I discussed the evolution of American English and its treatment of collective nouns here. Andy Hall has weighed in with evidence showing “United States” being referred to in the singular prior to the war as well: “A similar search comparing the frequency of the phrases ‘United States are’ and ‘United States is’ reveals that, contrary to Foote’s assertion that the former was the preferred usage in the decades before the war, the two phrases were actually used about equally through the first few decades of the Republic. That began to change in the 1840s, when ‘United States is’ (shown in red) began gradually to pull away from ‘United States are’ (in blue) in printed usage. By the beginning of the war (shown here as a green bar), ‘United States is’ was solidly more common in usage–though not greatly so–than ‘United States are’ ” There were still disagreements about the usage even at the end of the 19th Century and into the 20th Century. See here and here.
This episode gets into the legacy of the war. Highly reconciliationist, it paints a picture of brotherly reunion after the war, for the most part.