"The Biden trip was principally an effort to calm nerves and to reassure countries of Central Europe that the U.S. was not losing interest and pursuing rapprochement with Russia at their expense," Kupchan said.

American officials have said some of the initial criticism was unjustified and based on a flawed belief that the U.S. was completely giving up on the plan.

Biden conceded the U.S. could have done a better job introducing the new plan. "Obviously it could have been done better," he told reporters in Prague. "But that's the reason for the trip. I think I set out on behalf of the president to convey to three central European allies that we're committed."

He said he was "absolutely convinced" after the trip that the three countries' governments and opposition leaders now "have no doubt about the commitment," adding that "missile architecture was more sort of a metaphor for 'are we committed?'"