When Finland and Sweden applied for NATO membership last spring, Russia’s reaction was negative but muted. It consisted only of words, not actions — in all likelihood due to Russia’s preoccupation with its war against Ukraine. Yet we should not assume that Russia will refrain from responding in the future. The Kremlin made its position clear years ago: there will be consequences from Finnish and Swedish NATO membership. Finnish president Sauli Niinistö offered one illustration of Russia’s approach in an interview in February 2022, recalling Vladimir Putin’s warning from 2016: “When we look across the border now, we see a Finn on the other side. If Finland joins NATO, we will see an enemy.” As we argue in a recent Center for a New American Security report, NATO’s forthcoming enlargement will permanently alter the European security architecture and erode Russia’s geopolitical position. Moscow will see these changes as a threat to its security and is likely to respond in ways that will pose challenges to NATO in both the short and long term. In the short term, the allies will need to counter Moscow’s attempts to undermine NATO’s position in the Nordic-Baltic-Arctic region, including through various gray zone tactics and more aggressive nuclear posturing aimed at compensating for losses in its conventional military capacity. In the long term, NATO must plan for a resurgent Russia, as the country will eventually reconstitute its conventional forces in the North and adapt its force posture in response to NATO’s presence in Finland and Sweden. With Finland and Sweden in NATO, Russia’s northwestern flank becomes more vulnerable. Its border with the alliance will then extend from the Arctic Ocean to the Baltic Sea, which will become almost entirely ringed by NATO countries. Concerningly for Moscow, alliance territory will expand near the strategically important Kola Peninsula in the north and move closer to Russia’s second-largest city of St. Petersburg, located on the Baltic Sea coast. Russia may suspect that the alliance will concentrate more military resources along the lengthy Finnish-Russian border. Additionally, Russia may perceive that it is riskier to carry out naval operations in the Baltic Sea or worry about threats to its Kaliningrad exclave — soon to be surrounded by NATO member states. The increased exposure of Russia’s military assets on the Kola Peninsula is particularly relevant to Russia’s threat perception. Lying just east of northern Norway and Finland, the Kola Peninsula is of central importance to Russia’s national security. It hosts Russia’s Northern Fleet, which includes ballistic missile submarines that guarantee the country’s second-strike nuclear capability as well as attack submarines and cruise missile-equipped surface vessels that would help Russia to interdict U.S. reinforcement convoys on their way to Europe. The damage done to Russia’s conventional military in Ukraine will increase Moscow’s reliance on its nuclear forces and therefore bolster the importance of the Kola Peninsula to Russian military planners. Additionally, the Kola Peninsula’s location near the Western terminus of the increasingly viable Northern Sea Route will further ensure that the region’s security is a vital Russian interest in the years to come. One illustration of how the Russian military looks at Finnish and Swedish NATO membership can be found in a Russian Defense Ministry publication from December 2022. The authors of an article dealing with current challenges to Russia’s military security in the Arctic note that even if Finland and Sweden’s accession is mostly a legal formality, as their relations with NATO have already been established, it must be seen as the most urgent challenge for Russia. They provide two reasons for this assessment. First, NATO troops, weaponry, and equipment may be placed in the territory of Finland and Sweden. Second, NATO could deploy “operational-tactical missile complexes” to Finland, which would create threats to both the military-industrial complex in the Arkhangelsk region and transportation infrastructure. The article argues that Russia should prepare for these possible risks by building up its forces in the area as well as planning long-range precision strikes against targets in Finland and Sweden. Already, we are seeing signs of a Russian response. Russian politicians and senior officials have long threatened to take “appropriate military-technical measures” if Finland and Sweden tried to join NATO. In December 2022, and again in mid-January, Defense Minister Sergei Shoigu indicated the types of changes that may be underway. Reforms planned for 2023-2026 will include the creation of an army corps in the Republic of Karelia near the border with Finland as well as the re-establishment of the Moscow and Leningrad military districts through the dissolution of the current Western military district. Shoigu stressed in December that the rationale behind these changes was “NATO’s desire to increase its military potential near the Russian borders, as well as to expand the alliance by adding Finland and Sweden”, making it necessary for Moscow to “take retaliatory measures and to create an appropriate grouping of troops in the northwest of Russia.” While Finnish analysts do not see these developments as dramatic, they demonstrate that despite being bogged down in Ukraine, Russia is shifting gears in response to changes to the Northern European security environment. Even though these proposals signal the intention to react to the perceived threat posed by Finnish and Swedish NATO accession, Russia will have limited ability to do so using conventional military means in the short term. Given its ongoing need to focus on the war against Ukraine, as evidenced by recent reports that it is shifting troops from other theaters, including from areas near Finland, Moscow will for the time being likely rely on gray zone tactics to undermine NATO’s position in the Nordic-Baltic-Arctic region. Recent events suggest these may include attacks on critical infrastructure such as pipelines, undersea cables, or oil and gas fields, as well as acts of terrorism against Western officials. Fears that Moscow will again weaponize asylum seekers, as it did during 2015 and 2016, have already prompted Finland to start to construct fences along its border with Russia. Cyberattacks and disinformation campaigns targeted at Sweden, Finland, and other countries along NATO’s bolstered northeastern flank are an additional possibility. Nuclear weapons will also assume greater prominence in Russian strategy until the country can reconstitute its forces, which could take a decade or more. In conjunction with Putin’s increasingly aggressive nuclear rhetoric following conventional military losses in Ukraine, Russia has stepped up its nuclear posturing in the High North. Moscow reportedly relocated several nuclear-capable bombers to the Kola Peninsula prior to last October’s nuclear drills, which included tests of all three legs of the Russian nuclear triad in the Arctic. As Russia attempts to meet the apparent increased threat in Northern Europe while its army remains tied down elsewhere, it is likely to double down on aggressive nuclear signaling in the region. Eventually, however, Russia will recover. And while its military weakness may be temporary, the changes to the European security architecture wrought by its invasion are not. In the long run, then, Moscow is likely to permanently adapt its force posture in response to NATO’s presence in Finland and Sweden. A recent Finnish research report argues that Russia will eventually react to Finnish and Swedish NATO accession by beefing up its forces near Finland, although not to the numbers seen during the Cold War. The author estimates that due to the losses Russia has suffered in Ukraine and the slowness of establishing new forces, it is likely that there will be no significant increase in military power in Finland’s immediate neighborhood before the 2030s. As Shoigu’s statements also illustrate, Russia will seek to increase conventional deterrence along its northwestern flank as soon as it has the capacity to do so, and it is reasonable to expect actions such as strengthening anti-access/area denial defenses around the Kola Peninsula as well as reinforcing the border with Finland near Saint Petersburg. Moscow is also likely to act more aggressively in Northern Europe after rebuilding its forces, if not sooner. More frequent provocative air exercises along the borders of Finland and Sweden or harassment of Western ships in the Barents and Baltic seas could seek to probe NATO defenses and intimidate the alliance, helping Russia to regain the advantage in the region. Moscow may focus these maneuvers on strategic choke points such as the Danish Straits and the waters around Gotland, Bornholm, and the Åland Islands. Demonstrations of military prowess may additionally serve as occasions to showcase Russia’s renewed great power status, which has been damaged by its poor performance in Ukraine. Such demonstrations would raise the risk of accidents that could spark a conflict. A renewed military threat to the Baltic states, which have long feared a potential Russian invasion, is also plausible in the long term. On the other hand, the accession of Sweden and Finland will make it easier for NATO to defend the Baltic states, thus increasing the alliance’s defense and deterrence in the region and contributing to regional stability in Northern Europe. Any new NATO infrastructure in Sweden and Finland — such as upgraded airfields, intelligence facilities, or most critically nuclear weapons — will only intensify Russia’s aggressive posture in the Nordic-Baltic-Arctic region. Finland has already announced that it will station F-35s in Lapland starting in 2026, and more frequent large-scale NATO exercises in Northern Europe are likely as the years go by. As Russia emerges from the war it started in Ukraine, these moves will further raise Moscow’s threat perception and cause it to dedicate greater attention to its Northwestern flank. The new round of NATO enlargement creates a need to manage both the short-term and long-term evolution of the Russian threat. In the short term, NATO — along with the European Union, national governments, private sector companies, and individual citizens — should plan to increase defenses and resilience against hybrid threats. NATO should further demonstrate its willingness to respond proportionally to hybrid attacks attributable to Russia. In response to the heightened Russian nuclear challenge, NATO should review its nuclear posture, including the role of nuclear weapons in extended deterrence and escalation management along with preparations for fighting in environments affected by nuclear fallout. At the same time, the alliance should plan for the conventional threat posed by a resurgent Russia in the Nordic-Baltic theater without taking steps that would be unduly provocative. The integration of the highly capable Swedish and Finnish militaries into NATO will itself bolster the alliance’s regional deterrence posture, but more can be done. This should include changing the alliance’s command structure, revising its contingency planning in the region, upgrading the Swedish and Finnish reinforcement infrastructure, holding new large-scale exercises in Northern Europe, and improving capabilities in the High North such as air and missile defense and intelligence, surveillance, and reconnaissance. It would also be prudent to establish a NATO Air Defense mission for the Baltic Sea region, building on the existing Baltic Air Policing operation. Allies should also work towards a comprehensive strategy for Northern European security that conceives of the region as a single theater encompassing not only the High North but also the Baltic Sea and North Atlantic. These efforts could include joint determination of regional capability targets within the NATO defense planning process, along with the pooling of Northern European allies’ maritime and aerial surveillance capabilities. It is also important to convince Poland and Germany to take a more active stake in the region’s security, encouraging them to embrace their identities as Baltic Sea countries. Finally, NATO should foster a sense of broader allied solidarity in Finland and Sweden to facilitate their transition away from an ingrained mindset of neutrality. Helsinki and Stockholm have long focused primarily on their own territorial defense, yet as new members of NATO they will have a responsibility to protect allied territory more broadly. To facilitate this adjustment, other NATO allies should encourage Finland and Sweden to make significant contributions to missions not only on the alliance’s eastern flank, but on its southern flank as well. Finnish and Swedish accession can dramatically increase regional stability. Yet the alliance cannot afford to ignore the accompanying risks. The steps suggested above will help ensure that NATO is ready for any response that Russia has in mind. Nicholas Lokker is a researcher in the Transatlantic Security Program at the Center for a New American Security. His work focuses on the politics of European integration and security, Russian foreign policy, and transatlantic relations. Heli Hautala is a career diplomat who is currently on leave from the Finnish Ministry for Foreign Affairs. She is an adjunct senior fellow for the Transatlantic Security Program at the Center for a New American Security.