For decades, NATO has expanded inexorably outward, taking on new members and new missions that have carried it far beyond its original mandate in Western Europe and deep into the former Soviet sphere.

But Russia’s intervention in Ukraine has sent shivers down the spines of Eastern European countries from Estonia in the north to Bulgaria in the south. NATO’s newest members have been left feeling vulnerable and wondering whether the world’s most powerful military alliance is truly committed to their defense.

Concerns have been especially acute in the three Baltic nations that were once part of the Soviet empire and now fear that they could be next on Russian President Vladimir Putin’s hit list.

NATO has long resisted placing much of a footprint in the Baltics, worried that doing so would jeopardize ever-precarious cooperation with Moscow.

Now that that cooperation is on life support, NATO announced this week that it plans to substantially boost its air, sea and ground presence in the Baltic states.

A look at how membership has grown in the alliance.

After meetings with U.S. Defense Secretary Chuck Hagel, Polish Defense Minister Tomasz Siemoniak said in an interview with Washington Post editors Friday that he expected a plan to dispatch U.S. ground troops to Poland, and likely the Baltics, to be announced next week.

The decision has been made on a political level, and military planners are working out details, Siemoniak said.

A Pentagon spokesman said in a statement Friday that the United States is “considering a range of additional measures” to bolster air, maritime and ground readiness in Europe. “Some of those activities will be pursued bilaterally with individual NATO nations. Some will be pursued through the Alliance itself. All of them will be rotational in nature,” Rear Adm. John Kirby said.

NATO has been deliberately vague about plans for the positioning of its ground forces in Eastern Europe, a strategy that is in part intended to keep Moscow guessing but also reflects the lingering divisions within NATO over how far to go in provoking the Russian bear.

Like Ukraine, the three Baltic nations — Estonia, Latvia and Lithuania have significant Russian-speaking populations, people who Putin has suggested should, by all rights, be living in Russia. But unlike Ukraine, those three nations joined NATO in 2004.

The decision to increase the NATO presence has brought some relief in the lightly defended Baltics, but also questions about why NATO did not act earlier to try to deter Russia with a more robust show of strength on its eastern flank.

“Of course, we always wanted to see a more permanent presence from our NATO allies here. But before, it was not considered so urgent,” Estonian Foreign Minister Urmas Paet said in an interview. “Now, the circumstances have changed.”

Paet said that as part of NATO’s renewed commitment to the Baltics, NATO warplanes would, for the first time, regularly police the skies from an Estonian air base. Other measures are still under discussion, he said, including the stationing of U.S. ground forces in his country — a development that Paet said he would welcome.

NATO is a mutual defense organization, meaning that an attack on one nation is considered an attack on all. But for years after the tiny Baltic nations joined the alliance, NATO stalled in developing plans for how to defend its newest members. The alliance also avoided training exercises in the Baltics, out of deference to Putin’s complaints that NATO was reaching too far into his orbit.

The defense plans were drawn up only after Russian forces entered Georgia in 2008, and major training exercises remained largely off the table until the recent crisis.

Kurt Volker, the U.S. ambassador to NATO under presidents Obama and George W. Bush, said the delay was a mistake and that the alliance still is not doing enough to deter Russian advances.

“It’s a reactive stance. We’re saying to the Russians, ‘You do more, and we’ll do more,’ ” Volker said. “Frankly, Russia’s not impressed by that.”

Other NATO members have openly campaigned for the alliance to seize on the current crisis to make a major and lasting statement in Eastern Europe. Poland has been the most outspoken, with Foreign Minister Radoslaw Sikorski calling for the alliance to permanently station 10,000 troops in his country.

“For a long time, we’ve been considering the prospect of NATO, post-Afghanistan,” said Siemoniak, the Polish defense minister. “Now we have an answer to this question. It is that NATO must be able to respond to what is happening in Europe.”

Russia has argued that any mass deployment of NATO forces in Eastern European would violate the 1997 Founding Act, which covers the terms of cooperation between Moscow and NATO. Polish officials say that with 40,000 Russian troops allegedly massed on Ukraine’s eastern border, that deal has been voided.

But that view is not widely shared in NATO, and the alliance has been careful to avoid doing anything that could give Russia a pretext for escalation. Germany, which has extensive economic ties to Russia, has led the push for restraint.

“If we go down the direction of military threats, it’s easier to call our bluff,” said Constanze Stelzenmüller, a senior fellow with the German Marshall Fund, who favors the use of stiff economic sanctions.

But Stelzenmüller said German officials understand what makes Eastern European leaders so nervous, given the seemingly erratic nature of Putin’s recent behavior.

“The logic that Putin seems to be operating under is not the same logic that led us to believe that we could cooperate with and have pragmatic compromises with Russia,” she said.

When Putin held a televised question-and-answer session Thursday, it was evident that NATO’s push into Eastern Europe still rankled.

“We were once promised in Munich that after the unification of Germany, no expansion of NATO would happen to the east,” the Russian president said. “Then it started to expand by adding former Warsaw Pact countries, former U.S.S.R. countries. I asked: ‘Why are you doing that?’ They told me, ‘It is not your business.’ ”

If NATO had expanded further still to Ukraine, Putin said, Russia would have lost critical access to the Black Sea.

But Ukraine did not join NATO, and now the country’s territorial integrity has been compromised.

Baltic officials say they do not believe that Russia is planning operations in their countries like the one in Ukraine, and they cite the threat of a NATO response as a key reason.

“Article Five is absolutely a red line,” said Andrejs Pildegovics, Latvia’s state secretary for foreign affairs, referring to the provision in NATO’s charter that guarantees collective defense. “All allies should have full protection.”

But there is no question that Baltic officials are deeply apprehensive about what could happen if Russia succeeds in breaking off even more of Ukraine, after annexing Crimea last month.

This week, the head of Latvia’s national security committee accused Russian agents of quietly surveying Latvian opinion on the country’s eastern border — behavior that mirrors the lead-up to the Crimean invasion.

About a quarter of Latvia’s 2 million people are ethnically Russian, but Latvian voters in 2012 rejected a referendum that would have made Russian the country’s second official language. In the nation’s Russian-heavy east, there have been periodic calls for greater autonomy from the capital, and officials are worried that those calls will now grow louder, courtesy of covert Russian backing.

“There’s a huge difference between Ukraine and the Baltics,’’ said Tom Rostoks, a political scientist at the University of Latvia. “But after Crimea, we got the idea that almost anything could happen.”

Ernesto Londoño contributed to this report.