From RAND Corporation:

The Islamic State has emerged as the world’s richest terrorist group, with estimated assets of $1 billion to $2 billion. Its sophisticated and strategically driven financial scheme is a key reason that U.S. officials say this fight could last years.

 

An examination of newly declassified financial documents the group created dating to 2005 reveals an organized criminal operation that is funded through rackets like protection, extortion and the co-opting of the region’s oil industry. This makes the group a self-sustaining operation, largely free of reliance on the largesse of wealthy foreign patrons. While airstrikes may disrupt the flow of oil and profits, they will not lead to the group’s financial ruin anytime soon. Based on our research, we estimate the Islamic State will bring in $100 million to $200 million this year. And that’s being conservative.