HOW DID THIS HAPPEN?

A McClatchy report on ISIL (ISIS or IS) had this remark from an expert:

“As an organization, ISIS has become the wealthiest militant group in the world, with assets in the low billions of dollars, and has developed an almost obsessive level of bureaucracy, account keeping and centrally controlled but locally implemented military-political coordination,” said Charles Lister, who tracks the group for the Brookings Institution center in Doha, Qatar.

Where did the group get all this money?