PORT-AU-PRINCE—A power struggle is brewing in Haiti as the man appointed prime minister shortly before the assassination of Haiti’s president this week said he—not the acting premier—should lead the Caribbean nation and was forming a government to that effect. Ariel Henry, a neurosurgeon who was named prime minister by President Jovenel Moise on Monday, two days before Moise was killed by a squad of gunmen in his home in the capital, Port-au-Prince, said he was now the highest authority in Haiti, not interim Prime Minister Claude Joseph. “After the president’s assassination, I became the highest, legal, and regular authority because there was a decree nominating me,” Henry told Reuters in a phone interview late on Friday. Henry had not been sworn in to replace Joseph at the time of the assassination, which has created confusion over who is the legitimate leader of the 11 million people in Haiti, which shares …