Europe’s annual crisis may come earlier than usual this year, experts suggest. Everyone is eyeing Wednesday’s election in the Netherlands, fearing a populist revolt. Shortly thereafter comes the French presidential election, raising the specter of a victory by the nationalist Marine Le Pen—and then France’s exit from the European Union. If Frexit follows Brexit, some fear it will put paid to the project of uniting Europe and shock the Continent’s long-sputtering economy.

Gloomy projections for Europe reflect a classic forecasting...