TAMPA, Fla. - Islamic State group militants used armed, commercially available drones to bedevil U.S. and coalition forces fighting in Mosul in 2016, a vexing problem that the four-star head of U.S. Special Operations Command said was the "most daunting" threat his operators faced on the battlefield last year.

"This last year's most daunting problem was an adaptive enemy who, for a time, enjoyed tactical superiority in the airspace under our conventional air superiority...