In the book of Daniel, Nebuchadnezzar's dream of a great statue composed of different metals (Daniel 2) is interpreted as a sequence of future kingdoms. While the identities of the Babylonian and Persian empires are relatively clear, interpretations of the bronze, iron, and iron-and-clay kingdoms vary significantly. What is the primary challenge in definitively identifying these subsequent kingdoms, and how does this ambiguity impact the overall message of the prophecy?