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ISIS is now thought to be the world's richest terrorist organization, with an estimated $2 billion in funds.
This makes ISIS wealthier than several sovereign countries, including Nauru, Tonga, and the Marshall Islands.
"They probably have some wealthy donors in those countries, but they are seizing hundreds of millions in Iraq, and they are taxing people for whatever they please, such as for being Christian," Matthew Levitt, Director of the Stein Program on Counterterrorism and Intelligence at the Washington Institute for Near East Policy told Business Insider. "Most of their money is coming from criminal activity, there is a tremendous amount of criminal enterprise throughout the ISIS organization."
As far back as March, Iraqi Prime Minister Nouri al-Maliki has accused Saudi Arabia and Qatar of openly funding ISIS as his troops were fighting them.
"I accuse them of inciting and encouraging the terrorist movements. I accuse them of supporting them politically and in the media, of supporting them with money and by buying weapons for them," he told France 24 television.
In Kuwait, donors have taken advantage of weak terror financing control laws to funnel hundreds of millions of dollars to various Syrian rebel groups, including ISIS, according to a December 2013 report by The Brookings Institution, which receives some funding from the government of Qatar.
"Over the last two and a half years, Kuwait has emerged as a financing and organizational hub for charities and individuals supporting Syria's myriad rebel groups," the report said, adding that money from donors in other gulf nations is collected in Kuwait before traveling through Turkey or Jordan to reach the insurgents.
"Everybody knows the money is going through Kuwait and that it's coming from the Arab Gulf," Andrew Tabler, senior fellow at the Washington Institute for Near East Policy, told The Daily Beast. "Kuwait's banking system and its money changers have long been a huge problem because they are a major conduit for money to extremist groups in Syria and now Iraq."
Ironically, Kuwait is a staging area for individuals funneling money to an ISIS organization that is aligned with whatever is left of the Baathist regime once led by Saddam Hussein. In 1990, the US went to war with Iraq over Hussein's invasion and occupation of Kuwait.
U.S. officials have stated that they are "eye-to-eye" with Saudi Arabia on the importance of stopping these donations. The Saudi government views the organization as a threat to the kingdom's security, and made private donations to ISIS illegal when it designated them as a terrorist organization in March.
When it comes to the U.S. reaction, Levitt said the American are working towards stopping the flow of cash, but that it's the local governments that will have the greater impact.
Levitt added that no one has the specifics on how much money ISIS and other terrorist organizations are getting from wealthy donors in American-allied gulf nations.
"Some don't see what is going on in Syria and Iraq as terrorism," he said. "They say 'just because you are an Islamist doesn't mean you're a terrorist.'"
If ISIS continues to stockpile cash - enough to maintain its status as the wealthiest terrorist organization in the world - it's possible that they would they would have the means to engage in international terrorism, something that the group and its supporters are already threatening online.
"ISIS is more extreme than Al-Qaeda in many ways," Levitt said of possible future ISIS attacks outside Iraq and Syria. "If they accomplish their goals in Iraq and Syria, and are capable of doing so, I don't doubt that they would."