Adding to the momentum was the decision last week by the Ukrainian government to hold talks on aligning with this group, called the Customs Union, rather than with the European Union. Two other former Soviet states, Kyrgyzstan and Armenia, have also committed to joining this group, a sort of Nafta of Eurasia.

“The main Russian point here is to formalize a zone in which Russia has preferential economic interests and privileges,” Alexander Kliment, a Russian analyst at Eurasia Group in Washington, explained in a telephone interview. “Russia has informally been trying to do that for the past 10 years. But the Kremlin wants a formal structure.”

Now, it has that structure. The decision by the Ukrainian president, Viktor F. Yanukovich, to halt talks with the European Union and turn to the Customs Union instead seems a pivotal moment. It also touched off protests in Kiev, illustrating how the choice was also about more than trade: The European Union deal was also supposed to help democratize former Soviet states and spread Western values.

Lost in the broader tug-of-war between East and West were the workaday advantages that the Russian-supported trade bloc is increasingly able to provide as more countries join.

The bloc’s larger population means companies that invest within the region, such as Ford Motor, which builds cars in Russia, have more potential consumers without crossing a customs barrier. Russia offers lower energy tariffs to members. Sergei Y. Glazyev, an economic adviser to Mr. Putin, has said Ukraine will save $9 billion yearly on energy.