Geopolitics can be more important than economics. Just look at Greece. On purely economic grounds, Greece should never have been admitted to the European Union in 1981 and might have been ejected from the eurozone months ago.

But what many European policy makers know—even if few articulate it—is that Europe will be increasingly vulnerable to Russian aggression if its links to Greece are substantially loosened. Greece is the only part of the Balkans accessible on several seaboards to the Mediterranean, and thus is a crucial...