The group seized control of Mosul, Iraq’s second-largest city, this summer. But its power base is in Raqqa, eastern Syria, where ISIS is now in control of more than half of Syria’s oil assets -- along with a number of oil fields in Iraq -- according to energy expert Luay al-Khateeb. Al-Khateeb says the oil is finding its way to the black market and could be making ISIS up to $3 million each day.
Source: Institute for the Study of War, Petroleum Economist, Platts
More than 11,000 people have traveled from abroad to fight in Syria and Iraq, officials suggest, although some have gone back home again. They align themselves with different factions, and sometimes change loyalties as groups merge, disband or change allegiances. Naturally, countries with bigger Muslim populations tend to send the largest number of fighters.
But some nations with relatively small Muslim populations have sent a disproportionately large number of jihadis. Finland, Ireland and Australia have the highest number of foreign fighters per capita, although Finnish security officials say a minority of Finns in their count went for humanitarian reasons.
CNN's Atika Shubert, Ivana Kottasova and Sean O'Key contributed to this report.