Eric Posner, interviewed on Freakonomics Radio:
The great lesson of Roman history was that, for quite a long time, the Roman Republic had a limited government. It was not run by kings or emperors, and it was also highly successful. By the standards of the time, it was wealthy and it conquered lots of places. It was a fantastic kind of role model for the Founders who were very ambitious for the U.S. but who didn’t want a king. But then the Roman Republic collapsed and was replaced by an emperor.
So the Founders, looking back, said, “We’d like to be powerful like the Roman Republic, and we’d like to imitate the constitutional structure of the Roman Republic to the extent that it was able to maintain both liberty while creating a powerful country. But we want to avoid the errors they made which paved the way for an emperor.”
Good interview, full of other insight from Posner, like the principle that limitation of presidential power might help America to be great, but often by over-stepping (or ignoring) that limitation presidents become great.
Posner’s paper here.