The Civil War was a significant force in the eventual dominance of the singular usage by the end of the 19th century.
ðə sˈɪvəl wˈɔːɹ wʌzɐ sɪɡnˈɪfɪkənt fˈoːɹs ɪnðɪ ᵻvˈɛntʃuːəl dˈɑːmɪnəns ʌvðə sˈɪŋɡjʊlɚ jˈuːsɪdʒ baɪ ðɪ ˈɛnd ʌvðə nˈaɪntiːnθ sˈɛntʃɚɹi.