It is a common trope to compare America to Rome. Economically America has almost nothing in common with Rome. Rome was a non-industrial economy mainly based on plunder or at least transferring products from the provinces to the central government and upper class in terms of taxation (in kind, e.g. Egyptian grain, and monetarily denominated). When the Empire stopped expanding new wealth, for the most part stopped being created. Perhaps there is a little bit of a comparison with 19th century America and expanding to the west in manifest destiny but in modern America economic growth is driven by technological change. Not conquest.
This is the rise and fall of civilizations, while expansion is possible in the sense of, the majority of people can "win" at their culture (or have a clear path to), a civilization is successful. In Rome, and America this usually means economic improvement. In Egyptian times, this meant falling in line with the religion. Carroll Quigley has a very nice framework for what is a civilization. Let's just say he was a very influential historian, mentor of one rather recent US president.
