“Three weak economies getting together and integrating: How much good can come out of it?” said Nargis Kassenova, the director of the Central Asian Studies Center at Kimep University in Almaty, who is part of a small but vocal opposition to the union in Kazakhstan. “Now, it is even worse because one is under sanctions and drifting away from the West,” Ms. Kassenova added, referring to Western economic sanctions against Russia.

Although the presidents of Armenia and Kyrgyzstan attended the signing ceremony and expressed an interest in joining, the missing guest at the party was Ukraine. That country’s previous government in Kiev tacked back and forth on whether it would join the European Union or the new Eurasian group, eventually choosing Mr. Putin’s offer and igniting a public uprising that ended up bringing down the president in February.

“We lost someone along the way, I mean Ukraine,” said Aleksandr Lukashenko, the president of Belarus. Mr. Lukashenko, who received a $2 billion loan and energy concessions from Russia just before the signing, acknowledged that the union as constituted was something less than he had anticipated. “Unfortunately, it is not the agreement that our partners originally announced,” he said, according to Belta, the official Belarus news agency.

He also called for economic union to be followed by political and military unity, a concept that Kazakhstan rejected.

“We are not creating a political organization; we are forming a purely economic union,” Bakytzhan Sagintayev, the first deputy prime minister of Kazakhstan and lead negotiator, said in an interview. “It is a pragmatic means to get benefits. We don’t meddle into what Russia is doing politically, and they cannot tell us what foreign policy to pursue.”