VILNIUS, Lithuania—Vice President Biden’s we’re-all-in-this together tour of Eastern Europe, a two-country stop that concludes here Wednesday, has highlighted not only the growing regional anxieties over Russia’s designs on Ukraine but also how much work the Obama administration has to do to convince allies of its support.

At its core, Biden’s message here and in Poland was a simple reaffirmation of what one senior administration official called a “bedrock commitment”: The United States would honor NATO’s basic premise that if one member nation is attacked, all would come to its defense.

“President Obama and I view Article 5 of the NATO Treaty as an absolutely solemn commitment which we will honor,” Biden said in an appearance with Estonian President Toomas Hendrik Ilves, referring to the collective self-defense clause in the agreement. “We will honor.”

A timely message, perhaps, as Russian President Vladimir Putin celebrated Crimea’s return to Russia through an annexation the Obama administration and its European allies have called illegal. But having to deliver the message at all is a measure of how uncertain Eastern Europe remains as it confronts an expansionist Russia - and how uncertain many leaders here are of the Obama administration’s intentions to help.

In some ways, the anxieties are the inevitable result of America’s military retrenchment after more than a decade of war.

Nations through history have seen their power tested in post-war periods - from Israel leaving the Sinai after the hopeful Camp David peace accords and into a war in southern Lebanon a few years later to the U.S. experience in Korea soon after World War II.

For Obama, who will meet with European leaders next week, the concerns here are also rooted in policy, tone, and contrast with Putin’s brash nationalism.

Obama believed on taking office that it was his predecessor’s go-it-alone approach, particularly in Iraq, that worried America’s traditional allies in Europe and beyond.

The United States had become unpredictable under George W. Bush, the president argued early in his administration, and re-energizing alliances such as NATO, the Group of Eight and Group of 20, and a variety of Asian regional forums would persuade allies that the new administration intended to act in partnership, not on its own.

To a degree, it has worked, perhaps most vividly in the international military effort that helped depose Moammar Gaddafi as Libya’s ruler.

But as Putin has acted alone, those living in the old Soviet neighborhood are, again, looking for the assertive American hand that often defined its role throughout the decades of the Cold War.

As Ilves, a Swedish-born, American-educated former academic, noted in his appearance with Biden Russia’s intervention is “forcing us to reassess the assumptions of the past 20, 25 years.”

“The old idea of NATO, which I remember from 20 years ago, out of the area or out of business, predicated on a Europe that no longer has any threats - that, unfortunately, has turned out, with the actions we’ve seen against Ukraine, to no longer to apply,” he said. “The East-West relationship needs to be put on a new standing.”

At the grand Presidential Palace here in the lovely old district of Vilnius, Biden on Wednesday held a series of meetings with Lithuanian President Dalia Grybauskaite and Latvian President Andris Berzins, who flew in for the sessions.

“Russia cannot escape that the world is changing and rejecting outright their behavior,” Biden said. “And that there is a price to pay for naked aggression.”

Both leaders thanked the United States for its “decisive steps” following Russia’s intervention in Ukraine, and warned that the move had upended the “architecture” that has governed European security for the last few decades.

Biden delivered the same message thoughout - that the United States, deeply concerned by what Biden called Putin’s “land grab” in Crimea - will defend NATO as an institution and its member states if they become targets.

But in many ways, the visit itself was the message. Even Grybauskaite used the term “symbolic” to describe it before meeting with the vice president.

Beyond spine-stiffening pep talks - and some aspirational discussions about weaning Eastern Europe from Russian natural gas - Biden pledged very little of substance during the trip.

He highlighted recent U.S. military contributions to Poland’s defense, a dozen U.S. F-16 fighter aircraft. The administration also has increased by 10 aircraft its contribution to the Baltic air policing program, another step Biden noted.

But those planes will leave with the United States as early as next month when it rotates out as the NATO partner to the Baltic nations for the air policing effort. Poland will replace the United States in that role, casting doubt on whether the increased air patrol will continue beyond then.

The senior administration official traveling with Biden, who spoke on condition of anonymity to describe the meetings, said there is an assumption that once the United States rotates out of the program “an equal number of planes from allies will rotate in.”

“We have strong reason to believe that that is what will happen, that the continued elevated level of aircraft stationed in the Baltics will persist beyond the time of our deployment,” the official said. “If for whatever reason that changes, obviously, we’ll take a close look at making sure that that number stays elevated.”

On the issue of energy, Biden and the Eastern European leaders he met with discussed ways of both expanding their own energy sources and in helping Ukraine if need be.

Those ideas ranged over the short and long terms — from Poland easing regulations to allow faster development of its shale gas to future European pipelines that would lessen reliance on Russia’s.

Beyond reiterating a previous pledge to expand U.S. liquefied natural gas exports to the region, Biden offered ideas, future technical support and encouragement.

In Warsaw, where Biden posed before a banner celebrating the Solidarity movement , President Bronislaw Komorowski said he reminded the vice president of the 1980s and his own view of what contained Soviet aggression.

“What happened in those days that was helpful was a change in Russian oil and gas prices,” said Komorowski, a former Polish defense minister.

He then added, “Combined with the arms race.”

When it was his turn to speak, Biden said, “You have an ally whose budget is larger than the next 10 nations in the world combined.”

“So don’t worry about where we are,” he said.