In the Constitution "the United States" is treated as plural, but so is "the House of Representatives," "the Senate," and "Congress." Over time, usage changed in American English, so that these collective nouns became construed as singular. (In British English, collective nouns can still take plural verb forms.) "The United States" also went the singular route, but its path was complicated by the plural ending -s at the end of "States." (Civil War and Secession had NOTHING to do with this Grammer change, just like how we now write two ss in a row).You can put the in front of a plural word
We the People of the United States
Don't be pedantic. The issue is clear and decided unless modified by Constitutional Amendment.
ARE OR IS?; Whether a Plural or a Singular Verb Goes With the Words United States. (Published 1901)
Plural or Singular Verb Goes With "United States"