When American and foreign observers speculate about American decline, as they often do, they tend to draw parallels between the United States and the Roman Empire. But parallels between a modern, industrial, urban, nearly-monolingual nation-state and a loose collection of tribes with an agrarian economy are worthless.
A more apt comparison is between the America of the early 2000s and the Britain of the early 1900s. Is the United States repeating the United Kingdom's decline, but on a larger scale?