Although the Islamic State has lost nearly 98 percent of the territory it once controlled, the group is ripe for a comeback in Sunni-majority areas of Iraq and Syria. The main reason is its existing war chest, coupled with its skill at developing new streams of revenue. The Islamic State used to mostly rely on the territory it controlled, including cities and urban strongholds, to amass billions of dollars through extortion, taxation, robbery, and the sale of pilfered oil. But the group has proven that it is capable of making money even without controlling large population centers.
During the apogee of its territorial control in 2015, the Islamic State accrued nearly $6 billion, making it by far the wealthiest terrorist group in history. How could a militant group compile the equivalent of a nation-state’s gross domestic product? When it did hold territory, the Islamic State primarily generated its wealth from three main sources: oil and gas, which totaled about $500 million in 2015, mostly through internal sales; taxation and extortion, which garnered approximately $360 million in 2015; and the 2014 looting of Mosul, during which the Islamic State stole about $500 million from bank vaults.