NATO's 2 newest members would move the alliance closer to one of Russia's most sensitive military outposts

• Sweden and Finland have submitted their applications to be NATO's next members.
• Admitting Finland would extend NATO's border with the Kola Peninsula, a major Russian military hub.

In May, Finland and Sweden jointly submitted their applications to join NATO, a historic move prompted by Russia's attack on Ukraine.

Their likely accession to the alliance would end their decades of formal military non-alignment and rearrange the security environment in northern Europe.

"Finnish and Swedish membership in NATO will probably complicate military planning for Russia, especially when it comes to offensive military operations directed against NATO states in northeastern Europe," said John Deni, a senior fellow with the Scowcroft Center at the Atlantic Council and a research professor at the US Army War College's Strategic Studies Institute.

The addition of Finland would double NATO's land border with Russia from 750 to 1,600 miles and extend NATO's boundary with the Kola Peninsula, a critical part of Russia's security architecture and a region Moscow sees as a military bastion.

A submarine nest

The Kola Peninsula contains the largest concentration of nuclear weapons in the world. It provides access to the Barents and North Sea and has the only ports in the Russia Arctic that are ice-free all year.

The peninsula is home to Russia's Northern Fleet, which fields the majority of the country's nuclear-powered submarines. The fleet is a vital component of Russia's nuclear triad and second-strike nuclear capability.

In 2020, President Vladimir Putin elevated the Northern Fleet to an independent military-administrative district on the level of Russia's four other military districts — Western, Southern, Central, and Eastern — highlighting the importance of the Kola Peninsula and the high north to Moscow.

The Kola Peninsula has many military bases and installations that support the Northern Fleet and act as staging grounds for its operations in the high north, which NATO officials say is central to Russia's defense.

The Northern Fleet's most formidable assets are its roughly 20 operational submarines, many of them nuclear-powered. Its mainstays are the fourth-generation Borei-class and Yasen-class submarines.

The Borei-class nuclear-powered ballistic-missile submarines are some of Russia's newest and can each carry 16 ballistics missiles and up to 96 nuclear warheads. Two Borei-class subs are deployed with the Northern Fleet and three more are under construction and will join the fleet within the decade.

The first Yasen-class nuclear-powered guided-missile submarine was commissioned in 2013 and its newest variant, the Yasen-M, was commissioned last year. The Northern Fleet has two Yasens and three more will eventually join it.

The Yasens carry a mix of conventional cruise missiles that can strike land or sea targets — long-range weapons that NATO officials believe Russia would likely use against ports and other infrastructure.

The majority of the Northern Fleet's nuclear-powered submarines are stationed in the fleet's headquarters in Severomorsk in Kola Bay. There are also submarine bases in Zaozyorsk, which is only about 40 miles from Norway, in Gadzhiyevo on Olenya Bay, in Zapadnaya Litsa, and in Vidyaevo.

The Plesetsk Cosmodrome, which houses batteries of RS-24 Yars thermonuclear ballistic missiles, is also on the peninsula, as are a number of airbases that can support strategic bombers.

Nuclear-support structures

In 2012, Putin ordered the modernization of Russia's military arsenal, with priority given to its nuclear weapons. The very large concentration of such weapons in the Kola Peninsula led to a program of upgrade, expansion, and modernization of the region's naval and air force facilities.

According to a 2018 review of satellite imagery by The Barents Observer, a Norwegian outlet based just a few miles from the Russian border, Moscow is building 50 reinforced weapons bunkers to store long-range nuclear and conventional missiles on the peninsula's Okolnaya Bay, which is northern Russia's largest weapons depot.

The Russian Ministry of Defense is also expanding bases in the Kola Peninsula to better support Borei-class and Yasen-class subs.

New docking facilities and other infrastructure for subs and special loading and unloading facilities for nuclear warheads and ballistic missiles are being constructed in many of the peninsula's submarine bases, rectifying operational shortcomings faced by the Soviet-era Northern Fleet.

Russia is also modernizing one of the three airbases near Severomorsk. Upgrades to the Severomorsk-1 airbase would improve the Russian military's awareness of the region and extend its operational reach as the Arctic becomes more accessible.

Getting cold feet

Finland and Sweden already cooperate closely with NATO. Both are NATO Enhanced Opportunity Partners, the closest partnership a non-member can have with the alliance, and both are part of NATO's Response Force. NATO membership will deepen their cooperation.

"Russia will likely be concerned about NATO proximity to Russian forces in the Kola Peninsula during peacetime and conflict," Deni said, adding that Finland's membership might allow NATO "to improve its knowledge of Russian activities in the Kola Peninsula."

Despite the peninsula's importance for Russia, Deni was skeptical that Moscow would need "to increase its military deterrence in the north" if Sweden and Finland joined the alliance.

NATO currently has "a serious lack of offensively oriented military capability and capacity" in the region, Deni said. "A NATO threat against Russia really doesn't exist right now in northern Europe even if Finland and Sweden are included in the calculus."

"Instead, the threat posed by the West is constantly inflated by the Kremlin to boost its own standing domestically and to justify massive military expenditures," Deni told Insider.

Russian military planning for operations in the Baltic Sea and in the Arctic would be more complicated if Finland and Sweden joined NATO, however, as they "may be more willing and able to share information with NATO on Russian activities in these two regions," Deni said, echoing comments made by Gen. Christopher Cavoli, the top US Army general in Europe.

If Sweden and Finland joined NATO, the alliance would almost completely surround the Baltic Sea, Cavoli told the Senate Armed Services Committee on May 26.

That enclosure would present "a bunch of different dilemmas, almost geometric dilemmas, that Russia does not have right now," Cavoli said, "so it will be advantageous."

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.