For Europe and NATO, a Russian Invasion Is No Longer Unthinkable

President Vladimir V. Putin of Russia once proclaimed the dissolution of the Soviet empire “the greatest geopolitical catastrophe of the 20th century.” At the time, back in 2005, few expected him to do anything about it.

But then came Russia’s occupation of Abkhazia and South Ossetia from Georgia in 2008, its backing for Ukrainian separatists and the annexation of Crimea in 2014 and, most resoundingly, the full-scale invasion of Ukraine in 2022.

Now, with the rise of former President Donald J. Trump, who in the past has vowed to leave NATO and recently threatened never to come to the aid of his alliance allies, concerns are rising among European nations that Mr. Putin could invade a NATO nation over the coming decade and that they might have to face his forces without U.S. support.

That could happen in as few as five years after a conclusion of the war in Ukraine, according to some officials and experts who believe that would be enough time for Moscow to rebuild and rearm its military.

“We have always kind of suspected that this is the only existential threat that we have,” Maj. Gen. Veiko-Vello Palm, the commander of the Estonian Army’s main land combat division, said of a possible Russian invasion.

“The past few years have also made it very, very clear that NATO as a military alliance, a lot of countries, are not ready to conduct large-scale operations — meaning, in simple human language, a lot of NATO militaries are not ready to fight Russia,” General Palm said during an interview in December. “So it’s not very comforting.”

Anxiety over what experts describe as Mr. Putin’s imperial ambitions have long been a part of the psyche of states that border Russia or are uncomfortably close. “I think for Estonia, it was 1991” when his country’s alarm bells started ringing, General Palm said wryly, referring to the year that Estonia declared independence from the crumbling Soviet Union.

Just as Mr. Putin played down the Biden administration’s warnings that he was planning to invade Ukraine, Moscow has dismissed concerns that Russia is planning to attack NATO. The head of Russian’s foreign intelligence service, Sergei Naryshkin, said in an interview last week with the state-owned news agency RIA Novosti that they are part of a Western disinformation campaign to stir up discontent against Moscow.

Europe’s worry has been further fueled in recent months by Mr. Putin’s militarization of the Russian economy and huge spending increases for its army and weapons industry while, at the same time, some Republicans in Congress look to limit American aid to Ukraine.

“If anyone thinks this is only about Ukraine, they’re fundamentally mistaken,” President Volodymyr Zelensky of Ukraine warned at the World Economic Forum this month. “Possible directions and even a timeline of a new Russian aggression beyond Ukraine become more and more obvious.”

NATO maintains that it is prepared to defend the borders of all 31 member states which, collectively, have increased national defense spending by an estimated $190 billion since 2014, when Russia first invaded Ukraine. But that was the start of building back what had become a hollowed-out military network across Europe in the decades following the end of the Cold War, a process that still could take years, analysts say.

That “peace dividend,” as the shift was called, diverted trillions of dollars from military budgets to increase spending on health care, education and housing. Europe’s defense industry also shrank as demand for battle tanks, fighter jets and submarines plummeted.

In 2006, worried about being unprepared for conflict, the top defense officials from each NATO country agreed to spend at least 2 percent of their annual domestic output on their militaries. But it was not a requirement, and when military spending hit a low point in 2014, only three of the 28 member nations of NATO at the time met the benchmark. As of last year, only 11 countries had reached the 2 percent threshold, although a Western diplomat said last week that around 20 member states are expected to meet it in 2024.

The alliance will test its readiness in a monthslong military exercise — including 90,000 troops — that began last week in what officials are billing as the largest drill NATO has staged since the end of Cold War. That the exercise is a test of how NATO forces would respond to a Russian invasion has rattled nerves in border states, particularly the Baltics and Nordics.

“I’m not saying it is going wrong tomorrow, but we have to realize it’s not a given we are in peace,” Adm. Rob Bauer of the Netherlands, the chairman of NATO’s Military Committee, told reporters on Jan. 18.

Noting NATO’s plans for responding to its top two threats, he added, “That’s why we are preparing for a conflict with Russia” as well as what NATO considers its other top threat, terrorism.

The NATO exercise, known as Steadfast Defender 2024, is just one reason allies are approaching a “fever pitch” of concern that Russia could invade sooner than later, according to Christopher Skaluba, the director of the Transatlantic Security Initiative at the Atlantic Council in Washington.

He said Russia’s resilience in the face of Ukraine’s Western-equipped counteroffensive last summer had shown that Mr. Putin was “sticking around for the long term” and could redirect his economy and population to reconstitute the military within three to five years. “Just because it got all chewed up in Ukraine doesn’t mean they’re off the board for a decade or more,” Mr. Skaluba said.

And the prospect of Mr. Trump returning to the White House has forced Europeans to come to grips with the possibility that American support for Ukraine, or even its leadership role in NATO, could be drastically reduced as soon as next year, Mr. Skaluba said.

Taken together, “that is overcharging these broader concerns about Russia,” Mr. Skaluba said. “It’s just this unique mix of factors that is combining to make this long-held fear about Russian reconstitution, or a Russian attack on NATO, become just a little more tense than it has been for the last couple of years.”

The worries have become more pronounced just in the last several weeks.

In a Jan. 21 interview, Norway’s top military commander warned that “we are short on time” to build up defenses against an unpredictable Russia. “There is a window now that will perhaps last for one, two, maybe three years, where we will have to invest even more in a secure defense,” said the commander, Gen. Eirik Kristoffersen.

On the same day, President Sauli Niinistö of Finland sought to calm concerns prompted by reports that one Steadfast Defender scenario will test how NATO would respond to a Russian invasion of Finland. “None of the war games played over decades have been played out in real terms, and I wouldn’t overreact here,” Mr. Niinistö on a national radio program.

And this month, Sweden’s top military commander, Gen. Micael Byden, and its minister for civil defense, Carl-Oskar Bohlin, each warned that Sweden must be prepared for war.

“Let me say it with the power of office” and “with unadorned clarity: There could be war in Sweden,” Mr. Bohlin said at a security conference.

The warnings kicked up a storm of criticism from Sweden’s opposition party and pundits, who called the remarks scaremongering and hyperbolic.

“Swedes are wondering what the government knows that they do not know,” Magdalena Andersson, head of the opposition Social Democrats wrote in a follow-up opinion article. “Scaring the population will not make Sweden safer.”

Yet Sweden is poised to join NATO, following Finland’s accession last year, as both countries set aside years of military nonalignment over nervousness about Russian aggressions. And even as he described the commotion as “exaggerated,” Prime Minister Ulf Kristersson of Sweden made clear that Russia remains a top threat.

“There is nothing to suggest that the war is at the door now, but it’s clear that the risk of war has increased significantly,” Mr. Kristersson said in an interview with Sveriges Radio.

It hasn’t escaped Estonia’s government that the land mass that Russia seized in the initial days of its Ukraine invasion in February 2022 — before it was pushed back to the current front lines in eastern Ukraine — is roughly the size of the Baltic States.

“Their ambition is to restore their might,” said Col. Mati Tikerpuu, the commander of Estonia’s 2nd Infantry Brigade, which is based about 30 kilometers, or 18 miles, from the Russian border.

“We don’t think that this question is whether or not” Russia will try to invade, Colonel Tikerpuu said last month from his command headquarters at Taara Army Base. For many Estonians, he said, “It’s only a question of when.”

Johanna Lemola contributed reporting from Helsinki, Finland.