The Islamic State's looting of important archaeological sites in Syria has been well-documented over the past year, with the damage caused to ancient cities like Palmyra causing anger and outrage around the world. Unfortunately, attempts to assess the damage caused to these sites and others like them has been limited due to the conflict and chaos that has existed in Syria over the past four years.

Jesse Casana, an associate professor of anthropology at Dartmouth, has found a way to get around that problem, using extensive archives of satellite imagery to examine nearly 1,300 archaeological sites in the country. What his research found was surprising – while it was clear that there had been significant looting in areas controlled by the Islamic State, looting may have been even more widespread in areas controlled by opposition forces or the Kurdish People's Protection Unit (YPG).

“Most media attention has focused on the spectacles of destruction that ISIS has orchestrated and posted online, and this has led to a widespread misunderstanding that ISIS is the main culprit when it comes to looting of archaeological sites and damage to monuments,” Casana said in a statement, using an acronym to refer to the Islamic State.

In areas controlled by the Islamic State, Casana found around 21.4 percent of the sites showed some evidence of looting. Remarkably, this was lower than in areas controlled by opposition forces (26.6 percent) or by the YPG (27.6 percent). The lowest percentage of sites looted could be found in areas controlled by the Syrian regime, where around 16.5 percent appeared damaged by looting.

The report does make clear that some of this looting may have occurred while a different group controlled the area, given the shifting front lines of fighting over the past few years, and in some cases where it was unclear who controlled the site it was counted twice. It is also important to note that the Islamic State controlled the greatest number of the sites Casana was able to examine and that the largest number of looted sites were found in Islamic State-controlled territory.

Additionally, the majority of damage in YPG-held territory was classified as "minor looting," meaning that evidence had been found of  fewer than 15 holes had been dug with pickaxes for the purpose of finding artifacts. In contrast, 22.9 percent of looted sites in territory held by the Syrian regime and a remarkable 42.7 percent of those held by the Islamic State appeared to have evidence of "severe looting" – a classification that indicates the scale of the looting could only have been performed by a group of laborers operating heavy machinery.

Casana's findings were published in the journal Near Eastern Archaeology. In his research, the Middle East archeology expert gained unprecedented access to the archives of satellite data company Digital Globe from 2007 onwards, as well as older sources, including declassified imagery from the CIA-operated CORONA satellite that dates back to the 1960s.

While looting was illegal before the Syrian war, it still happened: Of almost 1,000 sites where Casana was able to evaluate for evidence of pre-war looting, some 25 percent appeared to have been looted (as the report notes, it appears some sites had been looted over a period of decades). Casana evaluated a similar number of sites for war-related looting between 2012 and 2015 and found 22 percent showed evidence of looting.

While these numbers may seem comparable, it's important to remember that these lootings occurred in just three to four years, and there is new evidence of looting at at least 100 sites that were previously pristine. Casana notes that as there are around 15,000 or more major archaeological sites in Syria, his data suggests that around 3,000 have been looted since the war began – a startling figure. "This represents a truly unprecedented expansion in looting activity and thus a dire threat to the region’s archaeological heritage," Casana writes in the report.

What explains the rise in looting over the past few years? The answer to that is simple of course: The chaos that has engulfed the country since the start of the civil war. This chaos is probably the reason that the areas controlled by the Syrian opposition and the YPG see so much minor looting, which is likely to be opportunistic and perhaps only occurred because of weak centralized authorities. The stronger authority exercised by the Islamic State and the Syrian government may have led to less looting of this type.

The cases of severe looting that occurred in areas controlled by the Islamic State and the Syrian regime are hard to ignore, however. Casana notes that a number of sites controlled by the Islamic State have an unusual pattern of damage, whereby large portions of mounded sites were simply removed en masse. It seems likely that this type of looting came with the tacit approval if not the direct involvement of the Islamic State itself, who have released footage of themselves destroying antiques with sledgehammers and bulldozers. The group have sometimes claimed theological reasons for the destruction, though experts say financial motivations may play a bigger role: The sale of antiquities is thought to have earned the militants tens of millions of dollars.

This sort of looting has only added to the anger directed at the Islamic State over the past year, but Casana found that it had also taken place in Syrian regime-held areas, such as the Roman city of Apamea, as early as 2012. While it is unclear whether looting of this type was state-sanctioned, the fact that it occurred so close to Syrian regime troops and often involved large numbers of people or heavy machinery suggests that, at the very least, officers turned a blind eye to it.

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