Col. Assimi Goita, who emerged Thursday as the head of the junta in power, worked for years with U.S. Special Operations forces focused on fighting extremism in West Africa. He spoke regularly with U.S. troops and attended U.S.-led training exercises, said officers from both countries, who spoke on the condition of anonymity because they were not authorized to discuss the matter publicly. Goita, who also received training from Germany and France, according to the officers, headed Mali's special forces unit in the country's restive central region, where fighters linked to al-Qaeda and the Islamic State have established a stronghold that...