MAIDUGURI, Nigeria — Boko Haram militants dressed as soldiers slaughtered at least 200 civilians in three villages in northeastern Nigeria, and the military failed to intervene even though it was warned that an attack was imminent, witnesses said on Thursday. A community leader who witnessed the killings on Monday said residents of the Gwoza local government district in Borno state had pleaded for the military to send soldiers to protect the area after they heard that militants were about to attack, but help didn't arrive. The killings occurred in Danjara, Agapalwa and Antagara.