How to lose an empire in nine easy steps. From Nick Giambruno at internationalman.com:

Debt can topple even the most powerful empires.
Whether it’s Rome, Spain, France, Britain, or the Soviet Union, excessive debt has played a critical role in their decline.
The typical pattern in these examples of collapsing empires is:
Stage #1: Empires achieve success and become overconfident.
Stage #2: Overconfidence leads to extravagant spending on luxuries and wars.
Stage #3: Empires finance this lavish spending by going into debt.
Stage #4: The debt grows to an unsustainable level and creates a crushing burden.
Stage #5: Empires finance the debt through taxation and currency debasement.