A year before American voters’ anger over the cost of living helped Donald J. Trump win the presidency, similar sentiments in New Zealand thrust in the nation’s most conservative government in decades. Now, New Zealand bears little resemblance to the country recently led by Jacinda Ardern, whose brand of compassionate, progressive politics made her a global symbol of anti-Trump liberalism. The new government — a coalition of the main center-right party and two smaller, more populist ones — has reversed many of Ms. Ardern’s policies. It has rescinded a world-leading ban on smoking for future generations, repealed rules designed to...