Next month, if all goes well, Ukraine will sign a "deep and comprehensive free-trade agreement" (DCFTA) and an "association agreement" with the European Union. Such accords do not represent applications for or endorsements of Ukrainian membership in the EU. Still, they would mark a milestone in cementing Ukraine's adhesion to European commercial and economic standards, which would immensely enhance Kiev's competitiveness and growth prospects. Most of all, if implemented, the accords would represent a first and decisive step toward real European integration.
Although some Ukrainian businesses would suffer from the increased market competition that would result from these accords, the biggest obstacle is Russia. To prevent Ukraine from signing the DCFTA at the upcoming EU Eastern Partnership summit in Vilnius, Moscow has of late pulled out all the stops. It has begun a trade war with Ukraine, threatening that if Kiev signs the agreement, it will be cut off from Moscow's cherished plans for a Eurasian customs union, face increased Russian protectionism and confront sharply higher prices for Russian energy.
Moscow might not stop there. Sergei Glazyev, an economic adviser to Vladimir Putin, said at the annual meeting of the Yalta European Strategy in September that Moscow would be within its rights to press for a fragmentation of Ukraine in response to Kiev's moves toward the West. The Ukrainian press also reported earlier this month that Russia is attempting to deliver new arms to its Black Sea Fleet, in violation of existing accords between Russia and Ukraine.
It is unusual that a limited though potentially significant trade accord could produce such passion and hysteria. But for the Russian government, Ukraine clearly arouses strong, even irrational emotions. These threats reaffirm the point that Mr. Putin memorably made to U.S. President George W. Bush in 2008: that in Moscow's view, Ukraine is not a nation, and that if it joins NATO, the country would be dismembered.
Nothing has changed since then. Moscow does not believe that Ukraine's independence, sovereignty and integrity are binding and recognized legal facts. Rather, they are flexible words that can be twisted to suit Russia's changing interests.
Furthermore, neither Mr. Putin nor his government—nor many ordinary Russians—can conceive of a truly independent Ukraine. Deep-rooted Russian historical mythology clings to the idea that Russia alone is the true heir of "Kievan Rus," the medieval state that arose in Kiev and spawned both modern Russia and Ukraine. Thus Russia is the only legitimate state-building factor in the Eastern Orthodox world.
Ukraine's move to the West would also destroy Moscow's hope of creating a Eurasian Customs Union and Economic Community, quashing the process, begun by the Kremlin, that Hillary Clinton has correctly dubbed "re-Sovietization." The formation of a Eurasian economic bloc is the cornerstone of Mr. Putin's foreign policy. The breakdown of this dream would erode his legitimacy and his claim to fame as the man who rebuilt the Russian Empire.
The irrationality of Russian thinking about Ukraine, and the stakes for both Moscow and Kiev, mean that Russian threats must be taken seriously and rebuffed openly by the West. In recent weeks Moscow has sought to aggrandize its influence in the Middle East and elsewhere at America's expense. Backing Ukraine during its moment of decision with Europe would be a good way for Washington to start pushing back. It is also key to ensuring the continuity of a 68-year peace in Europe—an unprecedented phenomenon that, thanks to Moscow, is now at risk.
Mr. Blank is senior fellow for Russia at the American Foreign Policy Council in Washington.