LAZDIJAI, Lithuania — At a border crossing with Poland, Ramunas Serpatauskas was waving down cars crammed with bags, blankets and often three generations of family members one recent evening.

A commander of the Lithuanian Riflemen’s Union, a civilian paramilitary organization, Mr. Serpatauskas was there to help refugees who had fled the war in Ukraine.

But the checkpoint, one of only two in a narrow 40-mile corridor as the crow flies between the Russian territory of Kaliningrad to the north and the border of Belarus to the south, is strategically important for another reason.

If Russia attacked, it would likely be here, Mr. Serpatauskas said. “Let the Russians try,” he added defiantly, “We will show them.”

The mood is tense in Lithuania, one of the three small Baltic States and NATO members that spent half of the 20th century under Soviet rule. Everyone here heard the recent warning by President Volodymyr Zelensky of Ukraine: “If we are no more, then God forbid, Latvia, Lithuania and Estonia will be next.”

Many Lithuanians worry that if Mr. Putin wants to test NATO’s resolve, the Baltics would be a place to start: an enclave of freedom whose only connection to other NATO allies is the 40-mile corridor Mr. Serpatauskas was patrolling, the so-called Suwalki Gap. It is widely seen as one of NATO’s greatest vulnerabilities.

“It’s an obvious strategic target for the Russians because you can cut us off from the NATO security system,” said Laurynas Kasciunas, chairman of the national security and defense committee in the Lithuanian Parliament. “We are in a sense a modern-day West Berlin.”

The analogy with West Berlin, an outpost of Western democracy surrounded by Soviet-controlled Communist Germany during the Cold War, has become popular. Many still hope that despite the missiles and tactical nuclear weapons pointing at them from Kaliningrad and tens of thousands of Russian troops deployed in neighboring Belarus in recent months, Moscow will not dare to invade — and risk war with NATO and the United States.

Lithuanian and NATO military commanders say that with Russian ground forces tied up in Ukraine and struggling more than expected, for now at least, an attack is unlikely.

But no one says it is impossible anymore.

“We do not panic but we should be prepared,” Mr. Kasciunas said. “Every family should have a plan.”

His wife has started stockpiling food (and told him off for eating one of the tins the previous night), and they have made plans for their children to evacuate to the countryside in case of an attack.

The day Russia attacked Ukraine, the Lithuanian government declared a state of emergency. Lists of schools and churches whose basements can double as air raid shelters have been published. Last week a military exercise took place on the Suwalki Gap. This week some 4,000 NATO troops will hold another exercise.

“Thank God we are part of NATO,” said Arunas Bubnys, director of the Genocide and Resistance Research Center of Lithuania. “If we weren’t NATO members, the situation would be very dangerous.”

NATO has had a permanent presence in all Baltic States since 2017, multinational battle groups that were deployed in response to Russia’s first invasion of Ukraine three years earlier and are meant to deter Russian aggression. But since the outbreak of war last month, the alliance has been moving from deterrence to activating its defense plans.

Col. Peter Nielsen is the Danish commander of the NATO Forces Integration Unit in Lithuania, which coordinates between the local military command and some 3,000 NATO troops currently in the country.

For now, he said, “No one is seeing a buildup along the border; on the contrary, troops and equipment have been pulled into Ukraine.”

But, Colonel Nielsen added, NATO is not taking any chances, especially since about 5 percent of Lithuania’s population is ethnically Russian. In Estonia and Latvia that share is 26 percent and 34 percent respectively, higher than in Ukraine.

“The geography of the Suwalki corridor is a reality,” he said, running his finger along a large map on his office wall. “Should they make an incursion into the Baltics under a pretext to protect their Russian minorities then NATO is engaged — and we’re at war with Russia.”

“We are revising and refining our plans,” he said. “We are shifting to prepare to defend.”

So are many Lithuanians. Gun sales in the country have surged. Shooting ranges are booked up for target practice and the Riflemen’s Union has seen its membership swell by more than 2,000 over the last two weeks. Tinned sardines and gasoline are flying off the shelves.

Alina Puriene, a respected professor of odontology, is ordering sandbags to seal the windows in her basement. She is lobbying her university to train doctors of all specialties in surgical skills and traumatic injuries.

“We need to be ready,” said Dr. Puriene, standing in her elegant townhouse in central Vilnius, the Lithuanian capital.

Lithuania, which has been occupied by Russians, Germans, Swedes and even the Ottomans in centuries past, has proved its fighting spirit more than once. When the Soviets took over the country’s occupation from the Nazis at the end of World War II, a dogged rebellion by freedom fighters resisted for eight years in the country’s southern forests. In 1990, Lithuania was the first Soviet republic to declare independence, a signal that riled Moscow, which tried — but failed — to stop it with tanks.

Last week, as the country marked 32 years of independence, the celebrations held a special meaning. On the streets of Vilnius, there were almost as many Ukrainian flags as Lithuanian ones. In towns and villages across the country, there were so many civilians swearing an oath to the paramilitary Riflemen’s Union that they ran out of uniforms.

“Freedom is fragile,” said Mr. Kasciunas, who went to spend independence day with soldiers in western Ukraine. “Ukrainians are fighting for us and for Europe, too,” he said. “By resisting, they are depleting Russia’s military resources. We need to use that time to mobilize for all future scenarios.”

Signs of mobilization were ubiquitous in ways big and small.

On a village square in the Kaisiadorys region in central Lithuania earlier this past week, the Duke Vaidotas mechanized infantry battalion was holding a camouflaging exercise near a primary school. During their break, a group of first graders excitedly gathered around a tank. One soldier handed his G36 assault rifle to a boy who weighed it in his hands.

“Our children know there is a war,” said Odilija Juzukoniene, one of their teachers. That morning two children had asked to learn a Ukrainian song in music class, out of solidarity. “For us in Lithuania, this war is personal.”

On social media and elsewhere, Lithuanians have bombarded their government with variations of the same question: “Will we be next?”

In the city of Druskininkai near the Belarus border, residents were spooked by the sound of nearby gunfire, prompting some to call city officials to ask: “Has the war started?” They had to be reassured that Belarusian troops were staging an exercise on the other side of the border.

But in the view of Gen. Jonas Vytautas Zukas, who until 2019 was the Lithuanian military’s commander in chief, “World War III has already started.”

“This is like 1939, when Hitler attacked Poland and no one believed it would lead to a world war,” he said.

That’s what worries Dr. Puriene, the odontology professor in Vilnius. At 66, she is old enough to remember Soviet oppression — the way her husband and she had to marry secretly in a church because religious services were banned, her mother’s stories about a friend who was killed in the anti-Soviet resistance, the families who were deported to Siberia.

“Western Europeans don’t understand Putin,” she said. “They have been living in a ‘postwar’ mentality since 1945. What we need is more of a ‘prewar’ mentality.”

She recalls doing target practice in school as a teenager and learning about anything from chemical weapons and surgery even though she studied dentistry.

“In Soviet times, all doctors were prepared as war surgeons” Dr. Puriene said. “I knew how to put on a chemical protection mask and how to treat a traumatic head injury. These days dentists just know how to deal with teeth.”

“We have a good life,,” she said. “We must be able to defend it.”

At NATO command, Colonel Nielsen said he had been humbled by the clearsightedness of Lithuanians since being posted here last summer. Ever since Russia’s war with Georgia in 2008, Baltic leaders had warned of Moscow’s imperial ambitions but too often their threat perception had been dismissed as paranoia in the West, he said.

“Thirty kilometers from here democracy ends, and they understand that,” Colonel Nielsen said. “When Communism fell, this country looked to the West to learn things: About the rule of law, about how to build democratic institutions. Now it’s time for us to learn from them.”

Tomas Dapkus contributed reporting from Vilnius, Lithuania.