Islamic State militants who last week captured the Mosul Dam, Iraq's largest, had one demand for workers: Keep it going.

Arriving in their Toyota pickup trucks, armed with Kalashnikov assault rifles and wearing a patchwork of military uniforms, robes and turbans, jubilant militants from the al-Qaida breakaway group told workers hiding in management offices they would get their salaries as long as the dam continued to produce electricity for the region under their control, according to a technician who was at the dam when nearly 500 militants drove off Kurdish troops.

Islamic State's rampage through northern Iraq has inspired terror as stories spread of beheadings and crucifixions. At the same time, its fighters are capturing the strategic assets needed to fund the Islamic caliphate it announced in June and strengthen its grip on the territory already captured.