David Cohen, the top counterterrorism official at the U.S. Treasury Department, argues that the U.S. military campaign against ISIS is beginning to cut into the group’s revenues.

Discussions of how the group funds itself necessarily rely on speculation and guesswork, but researchers are starting to get a better idea about the terror group’s finances. Eckart Woertz, a fellow at the Barcelona Centre for International Affairs, provides a useful summary of what’s known about the Islamic State’s financial lifelines. The groups revenues are likely somewhere between $1 million to $5 million per day. A U.S. intelligence official told the Guardian earlier this year that the groups assets swelled from around $875 million to over $2 billion after the fall of Mosul.

The biggest chunk of that comes from oil. ISIS is believed to control roughly six out of Syria’s 10 oil fields as well as several in Iraq. The group relies on smuggling networks, some of which date back to the days of the U.N. embargo against Saddam Hussein in the 1990s, to get the oil out of the country. A recent Financial Times article suggested that the black market oil is often refined at plants in Iraqi Kurdistan, meaning that ISIS is shipping its oil out through enemy territory.

Another chunk comes from looting and pillaging newly conquered areas—though the extent of this has sometimes been exaggerated—and from extracting funds from people in the territories ISIS controls in the form of taxes, extortion, and real estate. (Fleeing refugees leave behind property that can be rented out.) Then, of course, there are ransom payments from the governments willing to pay for the return of their hostages.

ISIS has also received funding from wealthy donors, many in Gulf countries, particularly early in its rise, though according to Woertz this never brought in as money as was commonly believed.

Some of these revenue streams may not be sustainable. ISIS continues to make territorial gains, but more slowly and at a much higher cost than it did a few months ago when it swept nearly unopposed across swathes of Iraq. Nowhere is this more evident than in the closely fought and bloody battle in Kobane, Syria, whose defenders have taken to referring to it as “Stalingrad.”

ISIS can still exploit those living in areas under its control, but are likely wary of the lessons of their predecessor organization, al-Qaida in Iraq, which faced a popular uprising in the middle of the last decade when locals grew fed up with its rule.

ISIS still holds thousands of hostages, but the grisly fate of James Foley, Steven Sotloff, and Alan Henning may result in fewer citizens of Western countries—whose governments will pay higher ransoms—heading to the region.

There’s also evidence that some of ISIS’s former backers in the Gulf are starting to reconsider as the group expands.

This makes ISIS’s oil all the more critical. Given that it may be up to a year before the Iraqi military is ready to launch a major counterattack against ISIS and the lack of coordination between Syria’s “moderate” rebels and their international backers, disrupting ISIS’s oil money may be the most productive thing the U.S. and its allies can do to halt the group.

Airstrikes, targeted sanctions, and crackdowns on smuggling routes are part of this effort. It’s also helpful that Washington’s Saudi allies seem willing to tolerate low oil prices. All of this could change if ISIS is able to capture more oil fields in Iraq, but many of these are in Kurdish territories in the North and Shiite areas in the South—not exactly a cakewalk.

Woertz notes that ISIS’s revenues, even if reduced from their current levels, are “a lot for a terror group, but not for someone who intends to run a state and rule over an extended territory.” As Hayes Brown points out, if you classify ISIS as the “state” it claims to be, its estimated $875 million in assets put it put it “roughly in the range of the budgets of the Democratic Republic of the Congo and Afghanistan, two countries not known for their massive expenditures.”

In other words, if ISIS plans to keep acting like a state, it may develop a pretty serious cash flow problem in the near future. If it switches tactics and acts more like a traditional terrorist network, more concerned with inflicting damage on enemies than building local institutions, it may be able to live off its nest egg for a while.