The al-Shabaab militant group has exploited Ethiopia’s internal turmoil to cross the border from neighboring Somalia in unprecedented attacks in the past few weeks, a military expert said, warning that the attacks are likely to continue. The deadly incursions into Ethiopia, Africa’s second-most populous country and long seen as an anchor of security in the Horn of Africa, are the latest sign of how deeply the war in the northern Tigray region and other ethnic fighting have made the country more vulnerable. Ethiopia has long resisted such cross-border attacks by al-Qaeda-linked al-Shabaab, ...