In the south-western corner of Lithuania, in the town of Vistytis, they know their enemy well.

On the other side of the border is the Russian exclave of Kaliningrad.

The area around the wire fence separating the two territories is highly sensitive, and we are told we can only film with an official escort.

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A few metres away, we can clearly see the huts and buildings that make up the Russian border post.

It’s so close that in the summer, locals say they can hear the guards on the other side singing.

The border post sits at the far end of an area known as the Suwalki Gap.

Previously referred to as “the most dangerous place on earth” and “NATO’s Achilles’ heel”, it’s a stretch of around 60 miles (96.56 km) on the borders of Lithuania and Poland.

It is also the shortest land route between Russian ally, Belarus, and Russian territory in Kaliningrad.

The concern is if President Vladimir Putin ever decided to escalate the war in Ukraine, Russian troops stationed at both ends of the Suwalki Gap could push in from the east and west simultaneously, potentially isolating Lithuania, Latvia and Estonia from the rest of NATO.

“The Suwalki Gap is a potential vulnerability as Kaliningrad and Belarus – now basically a military extension of Russia – could try to cut the Baltic States off from Poland,” underlines Linas Kojala, director of the Eastern Europe Studies Centre.

‘If Ukraine falls – we have a big problem’

Despite this, there’s no sign of panic in Vistytis; but for some residents, there is also no forgetting that Russia houses troops and reportedly nuclear weapons next door in Kaliningrad.

“We hear helicopters, maybe the tanks shooting,” says former military medic, Vilius Kociubaitis, who is now the town’s doctor.

He knows people who are nervous and tells us some have even packed their cars ready to flee to Poland, with its large military, if Lithuania is threatened in the future.

“We are afraid because if Ukraine falls, we have a big problem because the Russians will come to us from Kaliningrad and also from Belarus,” Vilius explains. “It’s a big risk.”

It’s a fear compounded by an announcement earlier this month by Belarus that around 9,000 Russian troops will be posted there as part of a “regional grouping” of forces, which it claims are needed to help protect its borders.

Lithuania is a NATO member, so it has the protection of the alliance, but the war in Ukraine has put people on edge.

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Volunteer militias have seen a surge in applicants willing to defend their country.

The Riflemen’s Union told us membership has risen from around 5,400 people at the start of the year to more than 8,000 now.

Others have joined the foreign legion in Ukraine.

Mindaugas Lietuvninkas spent the spring fighting the Russians around Irpin and is now in training for a return stint.

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He remembers what it was like to live under Soviet rule when Lithuania was occupied – never again, he vows.

“They [the Russians] want to come back, but they will not come back, at least I will be one of those who will not let them come here,” he tells me.

At the eastern end of the Suwalki Gap we visit another potential danger zone.

The growing number of Russian troops currently stationed over the barbed wire in Belarus could quickly cross the border if Mr Putin ever decided to attack.

‘We must be prepared all the time’

Lithuania’s government admits the “Suwalki Gap is a critical point” and while there’s no imminent threat of invasion, the risk remains.

“We must be prepared all the time,” said Vaidotas Urbelis, political director of the Lithuanian Ministry of National Defence.

“In the short term, I would say the risk and the Russian capabilities are lower, but the risk of escalation is higher and that’s why we have to prepare every moment for something to happen.

“We saw some concentration of Russian forces around us before the conflict.

“Now it’s less because everyone went to Ukraine… but even though the number of personnel decreased around us, most of the equipment is still here and the force needed is still here.”

Lithuania insists its military is prepared for potential Russian aggression.

NATO allies have also sent reinforcements and this month carried out one of the largest international military exercises in Lithuania involving around 3,500 troops.

For now, the country remains poised: watching its borders and strengthening its defences, readying itself to retaliate with full force to any potential attack.