According to diplomats involved in Europe’s policy toward Ukraine, so much bureaucratic energy had been invested in bringing Ukraine into the bloc’s so-called Eastern Partnership program that wishful thinking prevailed over hardheaded calculation, especially since Mr. Yanukovych and his allies repeatedly restated their commitment to embrace Europe in the months ahead of his about-face.

Brussels seemed to become locked inside its own minutely calibrated bureaucratic mechanism and shut out events that threatened to throw grit in the works.

When Stefan Fule, the union’s enlargement commissioner and one of its main interlocutors with Ukraine, visited Kiev in mid-November, Mr. Yanukovych told him clearly, according to officials in Kiev and Brussels, that he was having serious second thoughts about signing the long-planned association agreement with Europe at a summit meeting scheduled for the end of that month in Vilnius, the capital of Lithuania.

Mr. Fule, upon his return to Brussels, informed ambassadors privately of the possible setback but in a public statement issued Nov. 20 made no mention of any potential trouble and instead hailed the “determination” of Mr. Yanukovych and his allies.

Mr. Linkevicius, the Lithuanian foreign minister, said that Mr. Yanukovych had sent so many mixed signals about his intentions that it was impossible to know what he intended to do. From 2008, when the European Union began its Eastern Partnership scheme, Europe’s efforts to extend democratic values and free trade to Ukraine and five other countries in the former Soviet Union were based on calculations that turned out to be wrong.

“We knew that we were heading for some issues with the Russians,” said Carl Bildt, Sweden’s foreign minister and an architect of the eastward push, along with the Polish foreign minister, Radoslaw Sikorski. “But we have seen a different Russia emerging, during the last year in particular, than many anticipated.”

In its dealings with Russia, Mr. Bildt added, “We did two things wrong in retrospect.” The first, he said, was not to take issue with Moscow’s increasingly assertive claims since 2009 in its military doctrine that it had a right to protect Russians living outside the country, particularly in former Soviet lands. The second was Europe’s muted reaction when Russia started last year to block the import of Ukrainian goods and threatened severe hardship if Kiev signed the trade and political pact with Brussels.