One of the reasons that the euro zone has survived for as long as it has is the impressive ability of its leaders to postpone dealing with a series of questions that are as fundamental as they are inconvenient. Is it possible to sustain a monetary union without a fiscal union? (Probably not.) Is it possible to establish a fiscal union without genuine democratic consent? (We may yet find out.) And suddenly pressing: What is the relationship between the EU’s law and Germany’s?

For half a century the conflict hinted at by this last question could mostly be treated as theoretical. Then, last week, the German constitutional court (BVG) challenged the legality of the Public Sector Purchase Program (PSPP), the $2 trillion-and-counting quantitative-easing scheme first launched by the European Central Bank (the ECB) in 2015 to prop up the euro zone’s faltering economies, and restarted in 2019. The BVG’s ruling does not concern the ECB’s Pandemic Emergency Purchase Program (PEPP), a new, smaller quantitative-easing regimen under which the ECB will buy up to €750 billion in bonds to help stave off the effects of the mess that COVID-19 has left in its wake. But it may affect how the PEPP is run: Already widely considered inadequate for the task that lies ahead, the program may be hobbled by restrictions flowing from the BVG’s judgment, and that’s before another wave of German litigation tries to bring it down.