The Suwalki Gap is a beautiful, relatively flat, narrow strip of farmland about 100km long - with Russian troops at each end.
This is the border between Poland and Lithuania, with the Russian enclave of Kaliningrad at one end and the pro-Moscow republic of Belarus at the other.
If Moscow gained control of the corridor, the Baltic states would, catastrophically, be cut off from their other Nato allies.
The Suwalki Gap is named for one of the towns along the border.
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It is the only land connection between the Baltic states of Latvia, Lithuania and Estonia - all three of them are Nato and European Union members - and their Nato allies.
That means it provides the only land access from Poland that Nato forces could use if it became necessary to move into the Baltic states to defend them against Russian invasion.
The current Lithuania-Poland border has existed since the re-establishment of the independence of Lithuania from the Soviet Union on March 11, 1990. Until then, the identical border was between Poland and Lithuanian Soviet Socialist Republic of the Soviet Union, established in the aftermath of World War II.
The frontier has a long history of ethnic tensions between Poland and Lithuania, some of them lingering today in distrust between the two nations despite their Nato and EU memberships.
It has also long been a cause for concern about Russian activity, with Polish soldiers on duty 24 hours a day in watch towers along the Suwalki Gap - alert for any suspicious action.
Lithuania and Poland joined Europe's Schengen Area in 2007, which meant that passport checks were removed along the border.
After this, crossing the border is easy. However, there are still occasional Customs and police checks for smuggling of restricted goods.