Greed caused the financial crisis. Remember in the film Wall Street "Greed is good!" It may have been good for many years but it was the banks CDO scams that finally blew it all. Did I say scams? Well you investigate their structure and see what you think. An excellent explanation programme by the bbc here
BBC iPlayer - The City Uncovered with Evan Davis: Banks and How to Break Them
You might also like to watch this:
BBC iPlayer - Million Dollar Traders: Make Me a Trader
I would say that you never had an empire to loose in the first place (unless you count all the foreign countries whose affairs are being manipulated by the CIA and other nameless/blameless non accountable USA deniable outfits) No, if you want to know how to loose an empire you have to be British and have built huge wealth and an empire based upon opium trading, slavery and plunder. Now of course we are giving it all back in the form of unlimited immigration and benefit handouts to all comers.
Re Forex being the answer, please remember that you will need to be one of the 5%