The big question is, will there be a long run? Although Russia is in a deepening financial crisis, Mr. Putin appears to have decided that he can destroy the new Ukraine before it can fully establish itself and before an economic downturn destroys his own popularity.

The Russian president is stepping up the military and financial pressure on Ukraine. Over the weekend, the city of Mariupol came under attack from forces that NATO said were backed by Russian troops, undermining the pretense that the separatists are acting on their own.

Ukraine will defend itself militarily, but it urgently needs financial assistance. The immediate need is for $15 billion. But to ensure Ukraine’s survival and encourage private investment, Western powers need to make a political commitment to provide additional sums, depending on the extent of the Russian assault and the success of Ukraine’s reforms.

The reformers, who want to avoid the leakages that were characteristic of the old Ukraine, have expressed their wish to be held accountable for all expenditures. They are passing extensive legislation but also want the International Monetary Fund to go on exercising oversight.

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.