According to Jonathan Masters
The membership of Finland and Sweden is expected to strengthen NATO's eastern flank and its collective defense in northern Europe. Finland's entry into NATO will more than double the length of the alliance's borders with Russia, extending it by about 800 miles. In addition, Finland and Sweden will significantly expand the alliance's presence in the Baltic Sea and beyond the Arctic Circle.
Prior to the invasion of Ukraine in early 2022, Russia sharply criticized U.S. and allied leaders for post-Cold War NATO expansion into the former Soviet bloc and pressed them for mandatory security guarantees, including a ban on any new members. However, in recent weeks, President Vladimir Putin has said that Finland's and Sweden's membership bids do not pose a "direct threat to Russia," while warning the two countries that they could become bases for NATO forces or equipment. Swedish leaders have already said they do not want to host NATO assets; Finland has not yet expressed its preferences. Neighboring Norway, a member of NATO, allows allies access for exercises, but does not allow the deployment of permanent facilities or nuclear weapons.
Finland and Sweden have been cooperating with the alliance for almost thirty years and are considered by many to be good security partners. They joined NATO's Partnership for Peace program in 1994 and both have provided personnel for NATO-led operations in Afghanistan, the Balkans and Iraq. In 2014, they became two of six Enhanced Capabilities Partners and have since been working to improve the ability of their armed forces to operate alongside NATO forces. Sweden recently hosted more than a dozen NATO allies and Finland in the major maritime exercise BALTOPS 22 in the Baltic Sea.
NATO expects the proposed expansion will improve the security of the Baltic states, which have been members of NATO since 2004 and whose defense strategists have long been concerned that Russia could seize the Finnish and Swedish islands in the Baltic Sea, especially Gotland, and use them as bases to start attacks on their territory. Some Western military analysts say NATO will almost certainly need basing rights in Finland and Sweden to protect the Baltic states. Baltic leaders strongly support the accession of the Nordic countries and continue to pressure other members of the alliance to increase the size of NATO military forces in their countries.
The addition of Finland and Sweden is also expected to strengthen NATO's position in the Arctic, a region where Russia has invested heavily in commercial and military infrastructure. The accession of Finland and Sweden will lead to the entry into NATO of all Arctic states except Russia, which will allow the alliance to pursue a more consistent strategy in the region.