In Europe, a ruling by the German Constitutional Court that the European Central Bank (ECB) failed to adequately justify a program of asset purchases it began in 2015 is convulsing the political and financial scene. Some suggest it could lead to the unraveling of the euro. It may be difficult at first glance to understand why. Yes, the purchases were huge—more than 2 trillion euros of government debt. But they were made years ago. And the points made by the court are arcane. So how could a matter like this assume such importance?