HELSINKI - Finland became the 31st member of NATO on Tuesday, in a historic realignment of Europe's defences that drew an angry warning of "countermeasures" from the Kremlin.
Russia's all-out invasion of Ukraine last year upended Europe's security landscape and prompted Finland -- and its neighbour Sweden -- to drop decades of military non-alignment.
Finland's Foreign Minister Pekka Haavisto formally wrapped up the process by handing Helsinki's accession papers to US Secretary of State Antony Blinken, the formal keeper of NATO's founding treaty.
"With receipt of this instrument of accession, we can now declare that Finland is the 31st member of the North Atlantic Treaty," Blinken said, at a ceremony in NATO's Brussels headquarters.
NATO chief Jens Stoltenberg said that Russian President Vladimir Putin had "wanted to slam NATO's door shut. Today we show the world that he failed, that aggression and intimidation do not work."
"Finland now has the strongest friends and allies in the world," he said.
Joining NATO places Finland under the alliance's Article Five, the collective defence pledge that an attack on one member "shall be considered an attack against them all".
This was the guarantee Finnish leaders decided they needed as they watched Putin's devastating assault on Ukraine.
Finnish President Sauli Niinisto said: "It is a great day for Finland and I want to say that it is an important day for NATO."
But Moscow erupted in fury at the move, which doubles its land border with NATO member states to 2,500 kilometres, branding it an "assault" on Russia's security and national interests.
"This forces us to take countermeasures... in tactical and strategic terms," Kremlin spokesman Dmitry Peskov said.