As the Islamic State pushes through Syria and Iraq, the group has adopted an alarming tactic—it works hard to gain control over a region's water supplies and then uses access to food and water to control the local population.

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Over the summer, the group starved around 12,000 people in Amerli, Iraq, of water, food, and medicine for months, says CNN, before the siege was broken by the Iraq army. ISIS has also used its control over four dams that block the Tigris and Euphrates rivers—two of the most important rivers in the region—to “displace communities or deprive them of crucial water supplies,” says the Washington Post.

In August, the group took control of the Mosul Dam, blocking the Tigris. That dam produced electricity for the region, says Business Insider, and its “destruction would wash away Mosul in a matter of hours and send 15-foot high floods to Baghdad within three days.” That dam was soon reclaimed by Iraqi and Kurdish troops, with American support, says the Post.

In September, the group cut off the water to Balad Ruz, says the Post. “According to the town’s mayor... the militants lined the roads to the dam with improvised explosive devices, and the government was forced to hire trucks to bring potable water to the residents.” In other cases, ISIS loosed the water rather than held it back, drowning uncooperative towns.

Iraq and Syria are prone to severe water shortages, making control over water an even more important strategic factor, says the Guardian. “One could claim that controlling water resources in Iraq is even more important than controlling the oil refineries,” researcher Matthew Machowski to told the Guardian.