At this fall’s annual meeting with Russia experts and journalists in Valdai, Russia, Mr. Putin spoke of Ukrainians and Russians as one people, and he has threatened Ukraine with severe consequences if it signs the agreement with the E.U.

Last August, the Kremlin fired a warning shot, ordering tough customs restrictions against Ukraine and all but halting Ukrainian imports for a week. (This month, Ukraine halted Russian natural gas imports, but that seems to be more of a dispute over payments than tension over the E.U.)

On the E.U. side, East European members like Poland and Lithuania have been particularly keen to pull Ukraine out of Russia’s orbit as a further buffer against Moscow’s ambitions. Other members farther west, like Germany and France, already suffering from pronounced “expansion fatigue” and the euro crisis, have been less enthusiastic about Ukraine, especially given its rampant corruption and cronyism. (Ukraine ranks a dismal 144th out of 176 nations and territories on the corruption perceptions index compiled by Transparency International, an organization that monitors corruption around the world.) The United States, which in the first years after the breakup of the Soviet Union ardently courted Ukraine, has basically lost interest.

Nonetheless, there is a shared sense in the E.U. that its members carry responsibility for extending the values of democracy, human rights and rule of law to countries still quite distant from these values on the far reaches of the Continent, and so a formal signing of the association agreement — and the initiating of similar agreements with Moldova and Georgia — was scheduled for the European Partnership Summit in Vilnius. The summit meeting is the third under the partnership program, which was begun in 2009 to strengthen ties with former Soviet republics.

Basically, the E.U. is prepared to accept President Yanukovich’s corrupt business dealings and practices in the faint hope that working with the E.U. will have a remedial effect on Ukraine. But it has made a singular demand of Mr. Yanukovich: Release Ms. Tymoshenko or no deal.