“Islamic State’s capture of the Mosul dam gives it control over the water and electricity supply in northern Iraq.”

Source: www.bbc.com

There is a geography to insurgency.   This dam controls both the energy and water resources in the region, which gives the insurgents/rebels/terrorists greater local power.  On a related noted, this op-ed entitled, “How America Lost the Middle East” has plenty of foreign policy and geopolitical material worth discussing.