ISIS didn’t invent the tactic. Tunnels have been used in warfare for thousands of years, especially valued in asymmetric guerrilla war. Jewish rebels used tunnels against Roman legions; the Viet Cong did the same against U.S. troops in Southeast Asia.

Here in Karemlash, Iraqi Christian militias uncovered cramped earthen burrows used by ISIS fighters to hide from surveillance drones, artillery shells and U.S.-led airstrikes.

The tunnels were dug into a hill that covers an archaeological site for an ancient Assyrian city. The Islamic State also commandeered the Saint Barbara convent and dug deep tunnels through the floor of the chapel. The village marks the site where Alexander the Great fought the Persian emperor Darius in 331 B.C.