In 2006, we wrote a book with Addison Wiggin called Empire of Debt: The Rise of an Epic Financial Crisis. The idea of the book was as follows:
Empires are not the result of conscious thought; they happen when a group is large enough and powerful enough to impose itself on others. But empires are expensive. They are typically financed by theft and forced tribute. The imperial power conquers? steals? and then requires that its subjects pay "taxes" so that it can protect them.
The US never got the hang of it. It conquers. But it loses money on each conquest. How does it sustain itself? With debt.