Last week Mario Draghi, the president of the European Central Bank, declared that his institution “is ready to do whatever it takes to preserve the euro” — and markets celebrated. In particular, interest rates on Spanish bonds fell sharply, and stock markets soared everywhere.

But will the euro really be saved? That remains very much in doubt.

First of all, Europe’s single currency is a deeply flawed construction. And Mr. Draghi, to his credit, actually acknowledged that. “The euro is like a bumblebee,” he declared. “This is a mystery of nature because it shouldn’t fly but instead it does. So the euro was a bumblebee that flew very well for several years.” But now it has stopped flying. What can be done? The answer, he suggested, is “to graduate to a real bee.”

Never mind the dubious biology, we get the point. In the long run, the euro will be workable only if the European Union becomes much more like a unified country.

Consider, for example, the comparison between Spain and Florida. Both had huge housing bubbles followed by dramatic crashes. But Spain is in crisis in a way Florida isn’t. Why? Because when the slump hit, Florida could count on Washington to keep paying for Social Security and Medicare, to guarantee the solvency of its banks, to provide emergency aid to its unemployed, and more. Spain had no such safety net, and in the long run, that has to be fixed.