When ISIS first began, it relied on the assistance of other states in the region for funding: Saudi Arabia, Kuwait and Qatar.
Now, however, US intelligence officials and other experts contend that ISIS makes $3 million a day from oil revenue, human trafficking, theft and extortion.
Simply put, it is a finely tuned financial machine. Indeed, ISIS is perhaps the most organized and monetarily sound terrorist organization in history.
Between Iraq and Syria, ISIS controls as many as 11 oil fields. Through clandestine networks, it's selling this oil back to the very governments it is fighting.
Along with oil, ISIS is also smuggling weapons and antiquities.
Additionally, ISIS has gained a great deal of money by taxing local businesses and humanitarian organizations operating in the area.
ISIS is completely self-sustaining, which is quite terrifying considering its motives.
It's essentially like a giant fundamentalist mafia, as it engages in many activities similar to organized crime groups: extortion, kidnapping and ransoming, human trafficking (selling women and children into sexual slavery) and robberies.
The scary part is that, while most organized crime groups exist for the sole purpose of making money, ISIS also has territorial and religious ambitions.
It is now apparent that in order to defeat ISIS, the United States and its allies must cut off the group's funding and access to natural resources.
As long as ISIS has access to this kind of money, it will continue to gain more supplies and new recruits. Of course, this endeavor is much easier said than done, and will largely involve retaking territory that's now part of the Islamic State.
This infographic from the Washington Institute for Near East Policy provides an excellent overview of how ISIS obtains its funding and explains why this is one of the most important factors to consider in the fight against these extremists.
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