The essay written by John Bagot Glubb shows a fascinating look at the timeline of empires here are his theories:
-Empires lifespan is around 250 years

7 Phases of an Empire:
1. Age of Pioneers
2. Age of Conquests
3. Age of Commerce
4. Age of Affluence
5. Age of Intellect
6. The Age of Decadence
- Decadence is marked by:
-Defensiveness
-Pessimism
-Materialism
-Frivolity
-An influx of foreigners
-The Welfare State
-A weakening of religion.
(a) Decadence is due to:
- Too long a period of wealth and power
- Selfishness
- Love of money
- The loss of a sense of duty.
7. Age of Collapse
With this knowledge on hand, its very clear to see the USA is definitely in the Age of Decadence but might be moving towards the Age of Collapse.
With the Age of Collapse, every empire's down fall is different and unique.
As for the USA i believe the down fall will be based on the monetary system. With the continual rise of Bitcoin, it seems poised to take over.