Unfortunately, just as democracies are slow to move, an association of democracies like the European Union is even slower. Mr. Putin is exploiting this.

It is not only the future of Ukraine that’s at stake, but that of the European Union itself. The loss of Ukraine would be an enormous blow; it would empower a Russian alternative to the European Union based on the rule of force rather than the rule of law. But if Europe delivered the financial assistance that Ukraine needs, Mr. Putin would eventually be forced to abandon his aggression. At the moment, he can argue that Russia’s economic troubles are caused by Western hostility, and the Russian public finds his argument convincing.

If, however, Europe is generous with its financial assistance, a stable and prosperous Ukraine will provide an example that makes clear that the blame for Russia’s financial troubles lies with Mr. Putin. The Russian public might then force him to emulate the new Ukraine. Europe’s reward would be a new Russia that has turned from a potent strategic threat into a potential strategic partner. Those are the stakes.