The United States is sending troops to Nigeria to help train the West African nation’s military in fighting extremism, Nigerian authorities said Wednesday, as a multifaceted security crisis spreads in the north of the country . The American personnel won’t engage in combat or have a direct operational role, and Nigerian forces will have complete command authority, Maj. Gen. Samaila Uba, spokesman for Nigeria’s Defense Headquarters, said in a statement. “At the invitation of the Government of