One of Emmanuel Macron’s most endearing qualities is his unshakable faith in his own power to convince anyone of the truth of his beliefs. Last November, the youngest-ever president of France tried to commemorate the 100th anniversary of the 1918 armistice by touring small French towns situated on the former front line to talk about world peace. It did not go well. The ordinary citizens he encountered were less interested in the history of the Great War than in voicing their anger at his economic policies—especially a recently announced increase in gas taxes—and seemed only to get angrier the more he assured them that things would improve. A week later, some 300,000 people, mobilized over the internet, donned yellow safety vests and began to set up barricades on thoroughfares across France. It was the first step in what has turned out to be a roiling, monthslong political crisis.

Mr. Macron’s rise has been astonishing. Unknown to the general public until 2014 and never before elected to political office, he smashed his rivals to win the presidency in May 2017. His party, founded just a year earlier, swept the June 2017 legislative elections, granting him a solid majority and wrecking the center-left Socialists and center-right Republicans (the country’s two traditional governing parties).

Mr. Macron seemed to represent—to coin a phrase—hope and change: change from the generally mediocre political class that has governed France for 30 years, hope that France might embrace market-based reform and provide a model for combating the populist wave sweeping the West.

Today, the hope is on life support, and the change has yet to be seen. France’s economy seems as stubbornly stuck in neutral as ever, with unemployment around 9% (and youth unemployment at 21%), government spending at 56% of GDP and debt rising. Mr. Macron has had the second-fastest drop in popularity of any French President.

What happened?