ISIS managed to earn $2 billion in 2014 alone.
ISIS buys bombs and pays fighters with the billions of dollars it makes from the oil fields, mineral mines, and banks under its control. ISIS also imposes taxes on the people living inside its territory in Iraq and Syria. The Group has ditched Al-Qaeda's old model of relying on rich donors in the Arabian Gulf. Instead, the Islamic State is a self-funded powerhouse.
ISIS subsidizes bread for the public, experts say. Soldiers earn $400 to $1,200 a month, plus a $50 stipend for their wives and $25 for each child, according to the Congressional Research Service. Highly skilled engineers and technicians can make upwards of $1,500 a month, according to an investigative team of UN researchers.
"The Islamic State is certainly the best financially endowed terrorist organization in history. That is particularly due to its ability to govern ungoverned spaces," said Andreas Krieg, a military scholar at King's College London in Qatar.
So how does ISIS pay for it all?
Taxes: $360 million-plus a year: A huge amount of the money comes from the 8 million civilians who live and work in territory taken over by ISIS soldiers. Everything in the Islamic State is taxed. ISIS says all taxes are zakat, Islamic religious alms similar to a Christian tithe.
But ISIS also extorts its residents at every corner. School children must pay monthly fees: $22 for elementary students, $43 for older kids and $65 for university students, according to congressional researchers. These are fees residents didn't have to pay before.
It costs anywhere from $200 to $1,000 to bribe ISIS guards when traveling on Iraqi roads into their territory, according to congressional researchers.
There's even a special tax on Christians, mafia-like "protection insurance" called jizyah. The Islamic State's official magazine proudly speaks of "imposing jizyah."
And if you try to leave ISIS land, say, to visit family? The Islamic State demands a $1,000 departure tax and temporarily claims all of your property -- just in case you plan to leave permanently, according to the Geneva Centre for Security Policy.
Oil: $500 million a year: ISIS has hijacked oil wells and refineries. But it doesn't have the expertise or equipment to properly manage it at full capacity.
Numbers are all over the place. But the best estimates say ISIS in 2014 was producing close to 50,000 barrels per day, less than half the region's potential. And it was still raking in up to $1.6 million daily, according to the UN.
International sanctions prevented ISIS from selling this oil legally, so it has resorted to unregulated underground markets. But that's not hard. ISIS tapped into long-established smuggling routes that were set up back in the 1990s. Back then, Saddam Hussein used them to circumvent United Nations sanctions.
ISIS draws crude oil from captured wells. It sells that to middlemen at a quarter of the market price. When oil was at $80 a barrel last year, ISIS was selling it at $20 a barrel, according to the Financial Action Task Force.
ISIS is selling oil to its enemies. Syrian dictator Bashar al-Assad needs oil to keep lights on his part of the country. U.S.-backed Syrian rebels need diesel to keep engines running. ISIS doesn't mind literally fueling its enemies for now, because its goal is to keep funding the expansion of the Islamic.
Last week, Russia publicly accused the Turkish government of tacitly allowing an illegal oil trade that brings $1.5 million of oil into Turkey every day. Turkey's president denied those claims.
While oil is still an important source of ISIS income, it's no longer number one, according to the U.S. Defense Department. The U.S.-led coalition airstrikes that started in September 2014 have severely crippled the ability for ISIS to transport and refine oil. 8,573 strikes have hit 260 "oil infrastructure" targets, as well as thousands of fighters, buildings and equipment, according to the Defense Department. In October, they helped the Iraqi army take back the huge Bayji refinery from ISIS.
Bank looting: $500 million to $1 billion: When ISIS gains new territory, it claims control of banks. In 2014, the Islamic State stole up to $1 billion from banks this way, according to the U.S. Treasury. "Importantly, though, this source of revenue is not renewable," a Treasury representative said.
But that one-time injection of money made ISIS the wealthiest terrorist organization on the planet. When ISIS overran the Iraqi city of Mosul last year, it seized an estimated $450 million in cash and gold from the central bank branch there.
Kidnapping ransoms: $20 million to $45 million a year: American journalist James Foley was kidnapped in 2012, and ISIS demanded $132 million. It wanted $200 million for two Japanese hostages, Kenji Goto Jogo and Haruna Yukawa. Neither country paid, and both were murdered on video.
Most countries stick to a UN resolution to not fund terrorists this way -- under any circumstances.
It's an agonizing decision to make. Don't pay, and family members watch loved ones die. Pay up, and you buy terrorists weapons that kill thousands more. Plus, you give incentive for them to do it again.
But some nations do pay ransoms. One by one, Foley's cell mates in Raqqa, Syria were released because their countries paid for them. French, Italian and Spanish hostages were freed, according to a New York Times investigation last year.
But the vast majority of people kidnapped and tortured aren't foreign aid workers and journalists. They're local Assyrian Christians and Yazidi people.
Yazidis live in the region and have their own religion. ISIS punishes them for not being Sunni Muslim. ISIS regularly grabs Yazidi family members, then demands $3,000 payments overnight, according to the Geneva Centre for Security Policy. ISIS has kidnapped hundreds of Assyrian Christians, sometimes releasing them for $1,700 each, according to AFP.
"We are always talking about the Islamic State as a terrorist organization. But we have to talk about them as a state. It's a state now," Kaldager said. "They have their own economy, border control, customs, taxation. These make it very hard to get rid of them in the near future."
