NATO's newest member means Russia's 'unsinkable aircraft carrier' will be even more isolated

• Finland's admission to NATO greatly extends the alliance's land border with Russia.
• Finland also extends NATO's border along the Baltic Sea, which has been called a "NATO lake."
• That further isolates Russia's Kaliningrad region, which is surrounded by NATO member countries.

On April 4, Finland officially joined NATO , becoming the alliance's 31st member.

The addition of Finland extends NATO's land border with Russia from 754 miles to 1584 miles , moves alliance territory closer to major Russian cities and military bases , and further isolates Kaliningrad, an important Russian exclave on the Baltic Sea.

Covering an area of twice the size of Rhode Island, Kaliningrad lies between Poland and Lithuania and is home to the port of Baltiysk, Russia's only Baltic port that does not freeze during the winter.

Kaliningrad also borders the Suwalki Gap , a critical 60-mile corridor that connects Poland to the Baltic countries. The Suwalki Gap is widely considered a vulnerability for NATO and is likely to be targeted early in a conflict with Russia.

Kaliningrad is a major military outpost , hosting Russia's Baltic Fleet and other forces, and has been called an "unsinkable aircraft carrier" that allows Moscow to project power deep into NATO and EU territory.

An asset and a vulnerability

In recent years, Russia has modernized and increased its forces in Kaliningrad. The region was further reinforced before Russian's invasion of Ukraine, with the brigade defending it upgraded to a division in 2020. However, some Baltic Fleet units were redeployed to Ukraine where they have reportedly suffered very high casualties.

Russia's Baltic Fleet is based in Baltiysk and is composed of warships — mostly corvettes and a number of support ships — infantry and armored units, and aviation and air-defense forces. Russia also stores tactical nuclear weapons in Kaliningrad and has conducted tactical and conventional missile strike exercises from the region.

"One can argue that Kaliningrad is sort of a fortress sitting on the Baltic, with lots of cruise missiles and other weaponry, so it remains threatening," Steven Wills, a researcher with the Center for Naval Analyses, said on a recent episode of the CNA Talks podcast.

Although Kaliningrad's strategically valuable location means that Russian forces there can threaten neighboring NATO countries, the region is also a vulnerability for Russia, Dmitry Gorenburg, also a researcher with the Center for Naval Analyses, said during the podcast.

Kaliningrad is surrounded by NATO members, making it easier to cut it off from mainland Russia in a conflict. To the north are Lithuania and its Baltic neighbors, which have small militaries and would likely be focused on defensive operations in a war with Russia, but Poland to the south has one of the alliance's strongest militaries.

St. Petersburg, one of Russia's most important port cities, is connected to the Baltic Sea through the Gulf of Finland, a narrow waterway bordered to the north by Finland and to the south by Estonia.

With the admission of Finland, which has considerable military assets , the gulf would become a chokepoint in a conflict, limiting Russian maritime transit and complicating efforts to resupply or reinforce Kaliningrad by sea.

The possible addition of Sweden to NATO would further isolate Kaliningrad and bolster perceptions of the Baltic Sea as what has been called a "NATO lake."

The addition of Sweden and Finland would create dilemmas "that Russia does not have right now as they sail forth from St. Petersburg and Kaliningrad, so it will be advantageous" to NATO, US Army Gen. Christopher Cavoli, the alliance's Supreme Allied Commander in Europe, told US lawmakers last year.

NATO's control over the Danish Straits — which connect the Baltic to the Atlantic and are bordered by Denmark, Sweden, and Norway — would allow the alliance to further limit the movements of Russia's Baltic Sea Fleet.

"You end up in a situation where the Russian Baltic Fleet can potentially be bottled up quite easily, in much the same way that the Russian Black Sea Fleet has been bottled up by the closure of the Bosphorus" following Russia's invasion of Ukraine, Gorenburg said on the podcast.

NATO control over the Baltic would also make it much harder for Russia's powerful Northern Fleet , which is based on the Kola Peninsula in the Arctic, to sail through to Kaliningrad's aid, though the Northern Fleet has long-range aircraft and missiles with which it could strike at NATO forces.

Russia and NATO are well aware of Kaliningrad's vulnerability, and both sides are thinking about the exclave as "being sort of like the Alamo, a bit more vulnerable than maybe the impregnable fort on the Baltic that some have discussed," Wills said.

Constantine Atlamazoglou works on transatlantic and European security. He holds a master's degree in security studies and European affairs from the Fletcher School of Law and Diplomacy. You can contact him on LinkedIn .