The jagged Polish-Lithuanian border spans about 100km from its most southerly point, where both countries share a border with Belarus, to the north where it meets the Russian exclave of Kaliningrad.
As the crow flies, the distance between the two points is 65km.
This area is what western military analysts refer to as the 'Suwalki Gap', named after the Polish city of Suwalki, located about 30km from the border with Lithuania.
The term 'Suwalki Gap' was first used by former Estonian President Toomas Hendrik Ilves in 2015, who had warned of the area’s strategic importance to the three Baltic States.
That same year, US General Ben Hodges, the then commander of US Ground Forces in Europe, described the gap as "one of the most volatile points on the world map".
The 'gap' is a narrow corridor of land connecting Poland and Lithuania, flanked by Belarus and Kaliningrad oblast.
"Kaliningrad is Russia's western most military outpost. It's a kind of an island within NATO territory right now," Dr Damian Szacawa, a Polish political scientist who focuses on Baltic regional security, told RTÉ News.
The Suwalki Gap, he said, "is quite an important region, existential for all three Baltic states from their perspective".
Western military strategists have long feared that Russia could one day try to seize the 'gap', by launching a two-pronged attack from Kaliningrad and Belarus. That would cut off Lithuania and, by extension, Latvia and Estonia, from the rest of NATO.
However, the topography of the region benefits its defenders.
Populated by hills, forests, lakes and rivers, the Suwalki Gap does not lend itself to an attack by a tank division. Some of the area’s terrain is boggy too.
Any tank columns would need to advance via the region’s two main roads.
The early weeks of Russia’s invasion of Ukraine, in the area north of Kyiv, showed how invading tank columns on main roads can be decimated by drones, and small but mobile anti-tank units.
Despite these geographical advantages for the defence of the Suwalki Gap, many western military analysts still view the area as a vulnerable point in NATO’s eastern flank.
Firstly, Kaliningrad is heavily militarised, although varying estimates put the number of Russian troops stationed there at about 12,000 - not a huge garrison by any means.
Russia stations Iskander ballistic missiles in the exclave - the same 4,000kg missiles with a range of up to 500km that Russian forces launch at cities in Ukraine, and which have destroyed blocks of flats, killing scores of civilians on numerous occasions.
Secondly, two military training grounds are stationed on the Belarussian side of the ‘gap’.
One is located 3km from the Polish border and another is near the town of Grodno, about 25km from the Polish border and just 1km from the Lithuanian border, according to Professor Kryzsztof Federowicz, a political scientist who analyses the region for Lublin's Institute of Central Europe.
"This creates a potentially high risk of causing an incident or deliberately building tension," he wrote in a 2023 essay on the Suwalki Gap.
Part of a large-scale military exercise held by Russia and Belarus in 2021, known as 'Zapad' and involving 200,000 Russian and Belarussian troops, took place close to the borders with Poland and Lithuania.
That year's 'Zapad' drills simulated an attack on western countries, raising concerns in Warsaw and Vilnius at the time about the vulnerability of the Suwalki Gap.
However, last weekend's 'Zapad 2025' drills, the first to be held since 2021, the year before Russia's full-scale invasion of Ukraine, were much smaller and held farther away from the Polish and Lithuanian borders than in previous years.
It may be a signal that Russia and Belarus are not gearing up for any conventional-style incursion in the area.
Since 2017, NATO has deployed four multinational battlegroups in Estonia, Latvia, Lithuania and Poland, and, in recent years, the Polish and Lithuanian armies have conducted joint exercises in the Suwalki Gap to test defence scenarios.
Earlier this year, Lithuania also announced that it would upgrade one of its two main roads intersecting its border with Poland – which runs from Vilnius to the Polish city of Augustów – so that the route would be suitable for transporting military equipment too.
Sweden and Finland’s membership of NATO, another consequence of Russia’s war in Ukraine, "limit the importance of the Suwalki Gap", said Dr Szacawa, head of the Baltic team at the Institute of Central Europe in Lublin.
It is approximately 150km from the Swedish island of Gotland to the coast of Latvia, making air and seaborne operations to resupply the Baltic States a viable option should Russia ever attempt to cut the Baltics in half through a land incursion.
Also, NATO's announcement last week, in response to Russian drone incursions into Poland on 10 September, to further bolster defences along the alliance's eastern flank marks another commitment from the military alliance to shift more resources eastwards and deter potential threats from Russia.
"Due its geographical location, the Suwalki Gap will remain a flashpoint in NATO-Russia relations," said Dr Szacawa, but added that a direct military confrontation in the area is unlikely given Russia's ongoing invasion of Ukraine.
Russia's actions in the Suwalki Gap region, he believes, will be limited to informational and psychological operations, designed to engage Polish and Lithuania forces and divert their resources to the area.
"Anxiety is what Russia has been trying to create in the societies of the region for years."
