"Countering the financial resources of the Islamic State, which has seized significant territory in Iraq and Syria and threatened to conduct attacks against the United States and its citizens, has become a significant national security priority for policymakers, including Members of Congress. By undermining the financial strength of the group, also known as ISIL [Islamic State of Iraq and the Levant] or ISIS [Islamic State of Iraq and Syria], policymakers seek to reduce its capability to conduct terrorist attacks, as well as to ultimately 'degrade and ultimately destroy' the group. This effort includes a comprehensive look at how the group generates revenue. While IS funding streams remain fluid, the group's largest revenue sources appear (based on open-source information) to include oil sales, taxation and extortion, and the sale of looted antiquities. Oil sales initially provided the majority of the group's revenue, but gradually declined as a percentage of overall IS profits due to an extensive campaign of airstrikes by the United States and coalition partners against oil and gas facilities used by the group. U.S. officials have noted that the Islamic State's financial strength depends not only on its income but also on its expenses, and the extent to which it is able to devote its resources to military operations. U.S. officials have stated that the Islamic State's decision to hold and govern territory is a financial burden for the group, and thus a vulnerability that the United States could potentially exploit by diminishing the group's ability to generate and utilize revenue. If the Islamic State cannot afford the expenses associated with governing its territory, some argue that the resulting public backlash would undermine its ability to rule."
CRS Report for Congress, R43980