To understand the geopolitics of the region, start with Kaliningrad. It is a bit of Russia that is cut off from the rest. I have coloured Kaliningrad yellow on this map and as you can see, it has a coastline so it is accessible to the Russian fleet, but otherwise it has no direct land connection with the rest of Russia.
The land connection is known as the Suwalki Gap (I’ve marked it red on the map). Russia has been trying to establish a solid route along it since the break up of the Soviet Union. For more detail of the history of the Suwalki Gap, read the Wiki entry.
The gap or corridor is named after the Polish town of Suwalki, and it the route had become strategically sensitive to Russia since Poland joined NATO in 1999, and even more so since the Baltic states – Estonia, Latvia, and Lithuania – joined NATO in 2004.
Belarus under its current regime has close ties to Russia. So the recent decision by Belarus to store nuclear weapons supplied by Russia has increased tensions in the Baltic States. They fear that if Russia decided to command the land connection between Kaliningrad and Belarus then the Baltic States would be isolated from the rest of Europe, and easy pickings.
Putin clearly wants to reestablish dominion over what was the territory of the USSR, so it is difficult to see how a negotiated settlement can be achieved.
