Science Fiction Based on the Fall of the Roman Empire

Science fiction based on the fall of the Roman Empire. What may be more important here, I think, is the way most of our ideas of democracy (and what happens to it) seem to be based upon the fall of the Roman Republic. More and more it seems to me that on some level U.S. democracy is ideologically infused with a sense of futurity that pulls it towards a collapse into empire—as if for many people it's not worth fighting against the encroachments of the state, the expansion of the executive branch, or the evacuation of actually existing representation in government because, well, it's bound to happen eventually anyway.