There is no aroma of chocolate outside the Roshen Confectionery factory in Lipetsk, Russia, today. The Ukrainian candy maker's machines remain idle, toasting no hazelnuts, wrapping no bars. Russian authorities seized the factory last Wednesday after freezing Roshen's Russian accounts.

The company is owned by pro-Western Ukrainian businessman-turned presidential candidate, Petro Poroshenko. This confectionery expropriation surprised no one given the stench of brutality coming from Moscow's invasion of Crimea and its asset seizures there.

By invading Ukraine, Russia's Vladimir Putin seemed to think he could reverse a trend toward Westernization by taking advantage of the European Union's extended period of distraction due to repeated economic crises. Indeed, the euro zone has been involved in a struggle to protect the euro's very existence. The irony is that Russia's actions in Ukraine may have, single-handedly, solved one of the EU's most fundamental economic problems: how to keep weak economies from bailing on the euro zone together.

Public skeptics of the euro zone—and I've been one—question its durability because of its propensity to destroy itself from within. Locking countries into a monetary union at the wrong exchange rate leaves slow-growing members in a toxic stew of uncompetitiveness and stagnation from which it takes years to escape. The fundamental weakness of the euro zone is that it relies on hard-hit economies—such as Greece and Spain—to endure austerity packages for years to try to raise productivity and cut labor costs enough to make the fixed exchange rate within Europe sustainable. The skeptics predict that eventually these smaller, weaker economies will just give up—unless Germany, with its booming exports, subsidizes them over the long term, a prospect the Germans vehemently oppose.
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But then the Russians invaded Ukraine. Suddenly, tolerating some years of austerity in order to be part of the euro zone doesn't seem so bad.

Russia's moves are particularly menacing to the many recent additions to the European Union that did time behind the Iron Curtain, such as Bulgaria, Lithuania, Poland, Latvia, Hungary, Romania, and the Czech Republic. These very same countries are next in line to join the euro zone, but they have resisted the required tough conditions such as meeting inflation, deficit, debt and interest-rate targets.

The political will of these countries to actually bite the necessary bullets to join the euro zone had been nonexistent—killed by the euro zone crises of the last several years. Yet following Russia's invasion of Ukraine, they're back. Poland, after years of delaying conversion to the euro, now says it's seriously considering it.

For European economic integration to succeed, the countries with the weakest economies must be willing to endure pain in order to remain in or join the euro zone. Of the 15 countries in the EU with unemployment rates above 9% in the latest Eurostat statistics, almost half share a border with Russia or Ukraine. Of the 13 countries with unemployment below 9%, only one does (Romania).

The Russians seem to have found the one thing the EU has been searching for all along: a lasting way to convince peripheral economies to endure the sacrifices needed to be part of the euro zone. All it took was a reminder of how bad things can get for countries on their own.

Contrast the news on the banking systems as an example. Last week, Europeans announced a deal on a comprehensive banking plan that will give the European Central Bank resolution authority over all banks in the euro zone. Local authorities will surrender some national autonomy in a crisis and that may be unpopular. 

Alternatively, the Russians announced that Ukrainian banks will be treated as foreign in Crimea and held to Russian laws. The currency will convert to the ruble, and no one in Ukraine knows for sure whether their assets are safe. Bank runs were widespread in the days before the invasion.

After snuffing out the Roshen factory in Lipetsk, Russians will not be able to buy Ukrainian chocolate. Those in other nations still can, though, and Roshen's product catalogue is still online. The first product listed is the Roshen Bitter 67% Cocoa, described as a chocolate "with a balanced sweet and bitter taste." That may not sound as appealing as a fine chocolate from Belgium, but any European would take it over an old Soviet-style Soya Bar.


Mr. Goolsbee, a professor of economics at the University of Chicago's Booth School of Business and strategic partner at 32 Advisors, was chairman of President Obama's Council of Economic Advisers from 2010-11.