There’s an established body of work that draws a connection between drought, resource scarcity, and conflict in general. In a 2013 article for The Atlantic, William Polk, a historian and former adviser to President Kennedy, noted a possible relationship between Syria’s civil war and devastating 2006-2011 drought. “As they flocked into the cities and towns seeking work and food, the ‘economic’ or ‘climate’ refugees immediately found that they had to compete not only with one another for scarce food, water, and jobs, but also with the existing foreign refugee population,” he wrote. “Formerly prosperous farmers were lucky to get jobs as hawkers or street sweepers. And in the desperation of the times, hostilities erupted among groups that were competing just to survive.”

In addition, a paper published earlier this year in the Proceedings of the National Academy of Sciences specifically connects a severe drought across the Levant to the Syrian conflict.

The case isn’t a direct one. “Before the Syrian uprising that began in 2011, the greater Fertile Crescent experienced the most severe drought in the instrumental record,” the authors write, arguing that the drought is connected to a long-term change in the climate in the Eastern Mediterranean. “For Syria, a country marked by poor governance and unsustainable agricultural and environmental policies, the drought had a catalytic effect, contributing to political unrest.” ISIS existed in different form, as the Islamic State of Iraq, prior to the outbreak of the civil war, but the collapse of the Syrian state, combined with the fecklessness of the Iraqi armed forces and government, allowed the group to expand its reach and influence, and declare a caliphate.