It's a little unrealistic to compare Rome to the United States, I think. Thing is, the world we live in isn't the same as it used to be. The world is a lot smaller now. It's much easier to have an international influence, even control areas across the globe, than it was way back when. Rome fell because it spread its empire too thin. It's a bit more difficult to do that when we can cross the globe, realistically, in give-or-take a day. Such global influence is hardly unrealistic, and the United States has been able to manage to control areas half way across the world since the early 1900's.

However, just because the United States can maintain such an "empire" doesn't make it any more right.