When she became the world’s youngest female leader in 2017 at 37, Jacinda Ardern ignited a wave of euphoria known as “Jacindamania” in New Zealand while thrusting the Pacific island nation into the global spotlight. In the six years since then, she has become an icon of the left and a champion for women in public life, bringing a compassionate and authoritative energy to the hypermasculine realm of world politics. But on Thursday, Ardern said she would leave office, an announcement that was greeted with shock throughout the nation of 5 million people. “I know what this job takes and...