The Islamic State’s systematic pillaging and demolition of cultural artifacts in the Middle East is the worst large-scale destruction of the region’s cultural heritage since the Second World War, according to recent reports.

The terrorists, who have struggled to finance an expensive war especially with falling oil prices and coalition bombing of ISIS-held oil fields, draw a significant portion of their needed income from the sale of precious historical treasures. For the jihadists such a situation is a win-win, since they derive substantial economic benefit from the looting as well as stripping an entire region of important ties to its religious and cultural patrimony.

The result has been devastating for the region, and many of its effects will be permanent, since once destroyed, monuments are gone forever. France Desmarais, director of programs and partnerships at the International Council of Museums, has said that “we’re faced with the largest-scale mass destruction of cultural heritage since the Second World War.”

“What started as opportunistic theft by some has turned into an organized transnational business that is helping fund terror,” said Boston University archaeologist Michael Danti, “It’s the gravest cultural emergency I’ve seen.”

Some two-thirds of the Islamic State’s annual revenue—or $600 million—is invested in their fighting forces. Every month about $20 million are needed to pay for the main unit of the group, made ​​up mostly of foreign fighters (Muhajireen), who earn in the neighborhood of $400/month. Another 15-20 million dollars are used to pay local fighters and auxiliaries.

The Caliphate spends tens of millions of dollars a year to buy ammunition and explosives, and a one-week offensive can cost a million dollars in ammunition alone.

To continue financing such an enterprise, ISIS is always looking for new sources of revenue, such as its recent forays into hostage taking. Nonetheless, the smuggling of antiquities is reportedly the second largest source of funding of the Caliphate, after the sale of petroleum.

According to a recent National Geographic study, ancient sites in ISIS-controlled areas were more likely to have suffered severe damage than those in other parts of Syria.

“This could be read as evidence of more organized, potentially state-sanctioned looting in those areas,” said Jesse Casana, a Dartmouth University archaeologist heading up an analysis of damage done to Syria’s cultural heritage.

Casana noted that the damage wrought by the Islamic State is exacerbated by pillaging throughout the entire area, brought on in part by the general lawlessness that often reigns.

“The extreme actions by ISIL have led to understandable outrage across the globe,” he said. “But our focus on ISIL has also led to some misunderstandings about the scope of the antiquities crisis in the region, and about who is responsible for it, and therefore how best to address it.”

Along with the direct looting done by the Islamic State, leaders of the Caliphate regulate the sales of antiquities done by others as well, exacting a tax of 20% on all such activity.

In a September memo from its General Committee, ISIS officials declared it forbidden “for any brother from the Islamic State to excavate antiquities or give the permit to anyone from the public without receiving a stamped permit.”

Still, such taxes make up only a small fraction of ISIS antiquities income. According to the seized memos, in the first four months of 2015, the Islamic State had received $265,000 in taxes on total sales of $1.25 million.

Despite a nearly universal condemnation of Islamic State trafficking in antiquities, relatively little has been done to stem the flow of relics out of Iraq and Syria. Laws regulating the sale and transporting of merchandise vary from country to country and smugglers have established paths around the spotty customs enforcement at international borders. Moreover, traffickers in valuable historical contraband are often willing to wait long enough for interest to die down moving their wares. The result is a vibrant trade in antiquities, reaching into the realm of billions of dollars a year.

The U.S. State Department has is offering a $5 million reward for information that “leads to the significant disruption of” Islamic State trafficking of oil and antiquities, but so far, virtually no dent has been made in the looting and smuggling.

For now, the terror will continue as long as ISIS has the funds to sustain it.

Follow Thomas D. Williams on Twitter @tdwilliamsrome