Finland officially became the 31st member of the North Atlantic Treaty Organization (NATO) on Tuesday, marking a major shift in the security landscape in northeastern Europe that adds some 1,300 kilometers (930 miles) to the alliance's frontier with Russia.
The Nordic nation's accession was sealed during a formal ceremony at the NATO headquarters in Brussels on Tuesday.
US Secretary of State Antony Blinken and NATO Secretary General Jens Stoltenberg were on hand as the Minister for Foreign Affairs, Pekka Haavisto, established Finland's accession.
"Finland has today become a member of the defense alliance NATO. The era of military non-alignment in our history has come to an end. A new era begins," the Finnish presidency said in a statement.
"Each country maximizes its own security. So does Finland. At the same time, NATO membership strengthens our international position and room for maneuver. As a partner, we have long actively participated in NATO activities. In the future, Finland will make a contribution to NATO's collective deterrence and defense," it added.
Finland's acceptance into the US-led security alliance presents a blow to Russian President Vladimir Putin, who has long sought to undermine NATO, and before invading Ukraine, demanded the bloc refrain from further expansion.
The invasion instead drove non-aligned Finland and Sweden to abandon their neutrality and seek protection within NATO, though Sweden's attempt to join the bloc has been stalled by alliance members Turkey and Hungary.
On the eve of Tuesday's ceremony, NATO Secretary General Jens Stoltenberg heralded the raising of the Finnish flag for the first time at the alliance's headquarters in Belgium, saying "it will be a good day for Finland's security, for Nordic security, and for NATO as a whole."
But Russia has warned that further NATO expansion will not bring more stability to Europe, and on Monday said it would scale up forces near Finland if the alliance sent any troops or equipment to the new member country.
"We will strengthen our military capabilities in the west and northwest if NATO members deploy forces and equipment on Finnish territory," Russia's Deputy Foreign Minister Alexander Grushko told Russian state news agency RIA Novosti.
Kremlin spokesperson Dmitry Peskov reiterated that Finland's accession will force Moscow to "take counter-measures to ensure our own security, both tactically and strategically."
Prior to Tuesday, Russia shared about 1,215 kilometers (755 miles) of land border with five NATO members. Findland’s accesion more than doubles NATO's land border with Russia.
Finland's NATO membership guarantees the northern European nation access to the resources of the entire alliance in the event of attack.
It includes the protection offered by NATO's Article 5 principle, which states that an attack on one member of NATO is an attack on all members. It's been a cornerstone of the 30-member alliance since it was founded in 1949 as a counterweight to the Soviet Union.
NATO membership also better integrates Finnish forces in training and planning with NATO allies.
The country is no stranger to working with NATO, with its troops regularly participating in NATO exercises under a partner status.
The Finnish Defense Force also operate some of the same weapons systems as other NATO members, including US-made F/A-18 fighters, German-designed Leopard main battle tanks and K9 Howitzers used by Norway and Estonia among others.
Helsinki has also signed on the F-35 stealth fighter program, which will allow its air force to work smoothly with NATO members including the US, UK, Norway, Italy, Canada, Poland, Denmark and the Netherlands.