In the picture
With Finland's accession to NATO -and Sweden's in a brief period of time if things evolve as expected- all Baltic countries will be members of the Alliance except for Russia, who in comparison has narrow access to it: the region of Saint Petersburg and Kaliningrad.
As of April 4th, 2023, Finland has officially become NATO's 31st member. After a process lasting almost a year (Finland's application was formally made in May 2022 after the initial months of war in Ukraine), the Nordic nation that remained non-aligned throughout the entire Cold War decided to break with its traditional stance after Russia's invasion of Ukraine starting on February 2022. Thus, NATO now finds itself having twice the length of border shared with Russia (Finland and Russia share a total of 1,340 kilometers of border), but with a major strategic advantage for its defense efforts in the Baltic Sea.
So, what does this mean in terms of maritime security for the Baltic Sea?
The Baltic Sea region is of immense importance for the Alliance and the European continent. Such importance, almost inexistent during the 1991 to 2014 period, has been gradually increasing over the past decade, when the Russian annexation of Crimea took place. During the days of the Cold War, Baltic geopolitics looked quite different to what they do today. The region was divided into four main blocks: NATO (with Denmark and Germany in the West), the Warsaw Pact countries (Poland, Latvia, Lithuania, and Estonia), Russia in the Eastern coast and in Kaliningrad, and Sweden and Finland along the northern region. The naval forces of the Soviet Navy and the German Democratic Republic dominated the Baltic against the inferior NATO forces; leading many to think that should they decide to launch an attack, a great portion of Denmark together with the southern part of Norway and the northern coast of West Germany would be easily taken.
During the 60s, the idea popped up among both the Western and the Eastern blocks of establishing a 'sea of peace,' by virtue of which the Baltic would become an isolated sea free from any tensions, where any ship that wasn't from the local navies would not be able to penetrate (in other words, NATO ships would not be allowed to navigate through Baltic waters). The idea was quickly disregarded, however, by the (back then) almighty US Navy, which establishing the annual naval exercise BALTOPS in 1971 effectively prevented such thing from ever happening. The exercise, one of NATO's biggest joint exercises gathering every spring more than a dozen allied nations, constitutes a great opportunity for allied navies to bolster interoperability and joint military cooperation while ensuring freedom of navigation across the region is respected.
Now, decades later, and with the invasion of Ukraine strongly influencing security dynamics in the European continent and (especially) the Baltic region, Finland and Sweden's accession to NATO (the latter's will hopefully take place soon) is yet another step in the gradual transformation of the Baltic region, making it almost a 'NATO sea' with the exception of the Russian coast and Kaliningrad. This provides a strategic advantage for NATO in several ways, with special emphasis of two in particular.
First, Finland's (and Sweden's) official accession will translate in a boost of strategic superiority in the region. As of today, the most concerning security issues in the region for NATO are Lithuania, Latvia, and Estonia, the three Baltic republics with limited military capacity in comparison to Russia. If Russia hypothetically decided to launch an attack on them, NATO would need to react quickly and move there to protect them-something that some doubt would happen at this time. In such a case, the Baltic waters would turn into a vital line to send troops and supplies to the forces sent in their help. Furthermore, in trying to do this, Russia's Baltic Fleet based in Kaliningrad, with its anti-ship and surface-to-air missile systems, would have the chance to attack and disrupt such supplies, significantly reducing NATO's ability to provide protection to these countries.
With Finnish and Swedish territory now part of the alliance, this gap is significantly reduced. The island of Gotland (Swedish territory) in the middle of the Baltic could easily be transformed into an advanced base of operation from where to control Russian anti-ship and anti-aerial defenses in Kaliningrad. At the same time, both Finland and Sweden could be used as a land corridor to transport troops, weapons, and supplies in order to launch an amphibious assault against Russian forces from a much closer spot. Put simply, the geographical configuration of the two countries, penetrating the Baltic waters, could serve as a bridge from where to cross to the other side much easier and with better protection.
Secondly, Finnish accession will also enhance the NATO's strategic awareness in a region of vital importance for it. As NATO coastline in the Baltic keeps growing, so does the ability of the Alliance to better preserve freedom of navigation in these waters and contribute to the maintenance of peace. Furthermore, with several gas and oil pipelines and undersea cables running across the Baltic seabed (connecting the Nordic region to the mainland), the European continent is exposed to major risks of energetic and digital disruption. Given the vital importance that critical undersea infrastructure (hence the name) has for most European countries, having the possibility of increasing their protection through joint surveillance operations in the regions could also be a determinant factor in securing this infrastructure from hybrid warfare tactics of potential foes in the region.
In this matter, however, there is still much work to do, as the Nordstream incident proved in September 2022. But with the recent establishment of its Undersea Infrastructure Coordination cell, the Alliance proves to be on the right path. As stated by its commander, Lieutenant General Hans-Werner Wiermann, the center "will enable better coordination between key military and civilian stakeholders and with industry, on an issue that is vital to our security." With Finland and Sweden able to contribute in a more solid fashion to NATO's Standing Maritime Groups and providing the capacity to cover wider areas, Baltic maritime security is set to increase over the following months.
In sum, Finnish accession to NATO as made official on April 4th is a step in the right direction for Baltic security, as it provides greater cohesion and capacity of joint action against potential Russian hybrid tactics. In strategic terms, it enables a wider set of options for NATO to react against a potential Russian attack to any of the smaller Baltic states while further diminishing the influence of the Russian Baltic Fleet based in Kaliningrad. In more broader terms, it will also benefit the Alliance's capacity to face current maritime security challenges in the Baltic such as the protection of undersea critical infrastructure and freedom of navigation across sealines of communication.