I started my own thread on this because that other one was too long and too packed full of retarded comments. I had this thought back in 8th grade when I was still experimenting with the validity of concepts like social darwinism, self-interest as the fundamental motive of human beings, and a bunch of other pedantic ideas that really don't matter. What I eventually concluded is this: the United States won't paralell the historical power gains and losses of the Roman empire because, among many things, the Roman empire was constantly trying to expand its borders by sending detachments of its army into neighboring areas. The army was spread too thin and a bunch of barbarians took out rome. The United States admittedly pursues an interventionist policy, but does not proactively engage in war (anymore) for the purpose of gaining territory.
Furthermore, Rome became a benevolent dictatorship under Julius Caeser, but his successors didn't stay true to the benevolent aspect of things. Like every other despotic government in history, Rome floundered and dissipated. and whatnot.