By now we know who won Italy’s election on March 4: protest parties, especially the 5-Star Movement and the League (formerly the Northern League), which between them won half the vote. We also know who lost: the incumbent center-left Democratic Party of former Prime Minister Matteo Renzi, which secured only 19% of the vote. But it will take years to understand what this election and others like it in Europe really mean.

A few early guesses come to mind. Most important, it’s not at all clear that voters have fully embraced...