WASHINGTON — The clandestine American military campaign to combat Al Qaeda’s franchise in Yemen is expanding to fight the Islamist militancy in Somalia, as new evidence indicates that insurgents in the two countries are forging closer ties and possibly plotting attacks against the United States, American officials say. An American military drone aircraft attacked several Somalis in the militant group the Shabab late last month, the officials said, killing at least one of its midlevel operatives and wounding others. The strike was carried out by the same Special Operations Command unit now battling militants in Yemen, and it represented an...