by Priyanka Boghani

It’s been more than eight months since the United States mobilized a coalition and launched a campaign of airstrikes against the Islamic State of Iraq and Syria. Despite the strikes hitting more than 6,200 targets, it has been difficult to measure the coalition’s success against ISIS.

On Wednesday, a senior U.S. official offered one measure of progress when he told a French radio network that airstrikes had killed 10,000 ISIS fighters since the campaign began.

“We have seen enormous losses for Daesh [ISIS],” Deputy Secretary of State Anthony Blinken told France Inter. “More than 10,000 since the beginning of this campaign. That will end up having an effect.”

While that number seems substantial — given that estimates released by the CIA in the fall suggested ISIS had between 20,000 and 31,500 fighters — it might not be as significant as it initially appears. Even before airstrikes began, ISIS controlled major cities in Iraq and Syria, but the bombing campaign has yet to substantially slow its ability to launch new assaults and expand its territory. Just as troubling, analysts say, the death toll suggests ISIS is much bigger than Western intelligence first assumed.

“If [10,000 killed] is accurate, then that means all the estimates of ISIS strength from the fall were wildly inaccurate,” said Patrick Skinner, director of special projects at The Soufan Group, a New York-based security intelligence firm. “I’m pretty certain we haven’t gotten rid of either a third or half of their fighters.”

Estimates of the size of ISIS have varied widely. In addition to the CIA’s figures, the Syrian Observatory for Human Rights has estimated ISIS has 50,000 fighters in Syria alone. Others have put the size of the group’s fighting force as high as 200,000. By comparison, Iraq’s security forces are estimated to have approximately 84,000 between the military and federal police force combined, and Syria’s army is thought to have 125,000 regulars in April, according to The New York Times. But in Syria, ISIS is primarily attacking territory held by a hodgepodge of rebels and the Al Qaeda affiliate Jabhat al Nusra, whose numbers are smaller than the nation’s army.

Skinner pointed out that holding the amount of territory that the self-proclaimed Islamic State does requires many people — more than a force within the range of the CIA’s earlier estimates. According to the Syrian Observatory, ISIS currently controls half of Syria. It has held on to Mosul, Iraq’s second largest city, which it seized last June, and now controls more than half of Anbar province. And despite a bombing campaign that has cost the U.S. $2.44 billion, according to Defense Department figures, ISIS in recent weeks has also taken Ramadi in Iraq and Palmyra in Syria, and launched an offensive on opposition-held areas in Aleppo province in Syria.

Meanwhile, foreign fighters have continued flocking to ISIS. According to estimates released in May by the United Nations, 25,000 people have travelled to fight in Iraq and Syria, as well as in the conflicts in Afghanistan, Libya and Yemen. That figure includes those who have joined ISIS, Al Qaeda and other militant groups, but the report noted that ISIS “currently attracts most global foreign terrorist fighters.”

The number of fighters killed by the air campaign is an unclear measure of ISIS’ strength for another reason: Because of how broadly defined a fighter targeted by an air strike can be.

“If you consider the Obama administration’s definition of militant versus civilian when it does drone strikes, you are either a militant or a civilian with nothing in between,” said Aaron Zelin, the Richard Borow fellow at the Washington Institute for Near East Policy. “They define militant as any military-age male in the strike target area. So 10,000 could mean anything if they are transferring the same logic to what they deem [ISIS] fighters.”

The number of fighters killed in the U.S. fight against ISIS has not been publicized often. When the U.S. ambassador to Iraq told an Arabic TV station in January that 6,000 ISIS fighters had been killed, former Defense Secretary and Vietnam War-veteran Chuck Hagel dismissed body counts as a measure of progress, saying, “I was in a war where we did body counts and we lost that one.”

Better ways to measure success, Skinner suggested, would be looking at factors such as what the forces fighting ISIS are actually capable of achieving, or how many lives have been saved, or figuring out what kinds of social and political changes can lead to a long-term solution.

“The military does not want to get into body counts, because that never really works [as a measure],” said Skinner. “It’s kind of disturbing that a year after Mosul, we’re now doing the Vietnam-type body county to show success. When all the other metrics are bad, the last refuge is to show a body count.”