The European Union widened and deepened integration when it introduced the Single Market and the common currency, increasing the number of member countries from 12 to 28. After a quarter of a century, the 2008 financial and economic crisis opened a new chapter in the history of European integration. Prosperity was replaced by economic crisis and then long stagnation, with ramifications far beyond the economic arena.

For the first time, after more than half a century, some countries were almost forced to step out of the Union. History s most frightening migration crisis...